Xpra 3.0.6 failing to start xdummy server

[mjlbach]

Describe the bug
Attempting to start xpra fails with the following X server log:

❯ cat /run/user/1000/xpra/Xorg.:8.log
[    44.558]
X.Org X Server 1.20.5
X Protocol Version 11, Revision 0
[    44.558] Build Operating System: Linux 4.19.92 x86_64
[    44.558] Current Operating System: Linux nixos 4.19.106 #1-NixOS SMP Mon Feb 24 07:34:54 UTC 2020 x86_64
[    44.558] Kernel command line: BOOT_IMAGE=(hd2,gpt5)//kernels/454ar0mw1pjylarz448siildaxib5ml9-linux-4.19.106-bzImage systemConfig=/nix/store/v5lxzm71m86d2826ykd1v8yw3plak5fd-nixos-system-nixos-19.09.2166.ce9f1aaa39e init=/nix/store/v5lxzm71m86d2826ykd1v8yw3plak5fd-nixos-system-nixos-19.09.2166.ce9f1aaa39e/init loglevel=4
[    44.558] Build Date: 30 May 2019  06:28:09PM
[    44.558]
[    44.558] Current version of pixman: 0.38.4
[    44.558] 	Before reporting problems, check http://wiki.x.org
	to make sure that you have the latest version.
[    44.558] Markers: (--) probed, (**) from config file, (==) default setting,
	(++) from command line, (!!) notice, (II) informational,
	(WW) warning, (EE) error, (NI) not implemented, (??) unknown.
[    44.558] (++) Log file: "/run/user/1000/xpra/Xorg.:8.log", Time: Sun Mar  1 00:55:15 2020
[    44.558] (++) Using config file: "/nix/store/08nndxcwc4q1yy8i92gqjqkfy4qpksba-xpra-2.5.3/etc/xpra/xorg.conf"
[    44.558] (EE) Unable to locate/open config directory: "/run/user/1000/xpra/xorg.conf.d/1560"
[    44.558] (==) Using system config directory "/nix/store/az5r35q73m4rav5kh2bppvdxn86lf8rv-xorg-server-1.20.5/share/X11/xorg.conf.d"
[    44.563] (==) ServerLayout "dummy_layout"
[    44.563] (**) |-->Screen "dummy_screen" (0)
[    44.563] (**) |   |-->Monitor "dummy_monitor"
[    44.565] (**) |   |-->Device "dummy_videocard"
[    44.565] (**) Option "DontVTSwitch" "true"
[    44.565] (**) Option "AllowMouseOpenFail" "true"
[    44.565] (**) Option "AutoAddDevices" "false"
[    44.565] (**) Option "AutoEnableDevices" "false"
[    44.565] (**) Not automatically adding devices
[    44.565] (**) Not automatically enabling devices
[    44.565] (==) Automatically adding GPU devices
[    44.565] (==) Max clients allowed: 256, resource mask: 0x1fffff
[    44.565] (==) FontPath set to:

[    44.565] (==) ModulePath set to "/nix/store/az5r35q73m4rav5kh2bppvdxn86lf8rv-xorg-server-1.20.5/lib/xorg/modules"
[    44.565] (==) |-->Input Device "<default pointer>"
[    44.565] (==) |-->Input Device "<default keyboard>"
[    44.565] (==) The core pointer device wasn't specified explicitly in the layout.
	Using the default mouse configuration.
[    44.565] (==) The core keyboard device wasn't specified explicitly in the layout.
	Using the default keyboard configuration.
[    44.565] (II) Loader magic: 0x626d40
[    44.565] (II) Module ABI versions:
[    44.565] 	X.Org ANSI C Emulation: 0.4
[    44.565] 	X.Org Video Driver: 24.0
[    44.565] 	X.Org XInput driver : 24.1
[    44.565] 	X.Org Server Extension : 10.0
[    44.567] (II) systemd-logind: took control of session /org/freedesktop/login1/session/_32
[    44.567] (II) xfree86: Adding drm device (/dev/dri/card0)
[    44.567] (EE) systemd-logind: failed to take device /dev/dri/card0: Operation not permitted
[    44.568] (--) PCI:*(1@0:0:0) 10de:1b81:1462:3302 rev 161, Mem @ 0xde000000/16777216, 0xc0000000/268435456, 0xd0000000/33554432, I/O @ 0x0000e000/128, BIOS @ 0x????????/131072
[    44.568] (II) Open ACPI successful (/var/run/acpid.socket)
[    44.568] (II) LoadModule: "glx"
[    44.568] (II) Loading /nix/store/az5r35q73m4rav5kh2bppvdxn86lf8rv-xorg-server-1.20.5/lib/xorg/modules/extensions/libglx.so
[    44.569] (II) Module glx: vendor="X.Org Foundation"
[    44.569] 	compiled for 1.20.5, module version = 1.0.0
[    44.569] 	ABI class: X.Org Server Extension, version 10.0
[    44.569] (II) LoadModule: "dummy"
[    44.569] (WW) Warning, couldn't open module dummy
[    44.569] (EE) Failed to load module "dummy" (module does not exist, 0)
[    44.569] (II) LoadModule: "mouse"
[    44.569] (WW) Warning, couldn't open module mouse
[    44.569] (EE) Failed to load module "mouse" (module does not exist, 0)
[    44.569] (II) LoadModule: "kbd"
[    44.569] (WW) Warning, couldn't open module kbd
[    44.569] (EE) Failed to load module "kbd" (module does not exist, 0)
[    44.569] (EE) No drivers available.
[    44.569] (EE)
Fatal server error:
[    44.569] (EE) no screens found(EE)
[    44.569] (EE)
Please consult the The X.Org Foundation support
	 at http://wiki.x.org
 for help.
[    44.569] (EE) Please also check the log file at "/run/user/1000/xpra/Xorg.:8.log" for additional information.
[    44.569] (EE)
[    44.569] (EE) Server terminated with error (1). Closing log file.

To Reproduce
Steps to reproduce the behavior:

1. nix run nixpkgs.xpra
2. xpra start :200
3. cat /run/user/1000/xpra/:200.log
4. cat /run/user/1000/xpra/Xorg.:200.log