Libvirt: Failing to start VM in bridge mode

[tfmoraes]

Describe the bug

Libvirt VMs are not starting if network is in bridge. Using Virtual Machine Manager to start the VM It fails to start with this error message:

Erro ao iniciar o dominio: internal error: /run/wrappers/bin/qemu-bridge-helper --use-vnet --br=virbr0 --fd=31: failed to communicate with bridge helper: Transport endpoint is not connected
stderr=failed to parse default acl file `/nix/store/c3nr4r9ld8np3yz31circxfqipinfchq-qemu-5.2.0/libexec/../etc/qemu/bridge.conf'


Traceback (most recent call last):
  File "/nix/store/84z6w9fzflh3f3swm7lhllrryg2q633w-virt-manager-3.1.0/share/virt-manager/virtManager/asyncjob.py", line 65, in cb_wrapper
    callback(asyncjob, *args, **kwargs)
  File "/nix/store/84z6w9fzflh3f3swm7lhllrryg2q633w-virt-manager-3.1.0/share/virt-manager/virtManager/asyncjob.py", line 101, in tmpcb
    callback(*args, **kwargs)
  File "/nix/store/84z6w9fzflh3f3swm7lhllrryg2q633w-virt-manager-3.1.0/share/virt-manager/virtManager/object/libvirtobject.py", line 57, in newfn
    ret = fn(self, *args, **kwargs)
  File "/nix/store/84z6w9fzflh3f3swm7lhllrryg2q633w-virt-manager-3.1.0/share/virt-manager/virtManager/object/domain.py", line 1330, in startup
    self._backend.create()
  File "/nix/store/srvhsl43vbdyrydbanrrl559xdya2zcg-python3.8-libvirt-7.0.0/lib/python3.8/site-packages/libvirt.py", line 1353, in create
    raise libvirtError('virDomainCreate() failed')
libvirt.libvirtError: internal error: /run/wrappers/bin/qemu-bridge-helper --use-vnet --br=virbr0 --fd=31: failed to communicate with bridge helper: Transport endpoint is not connected
stderr=failed to parse default acl file `/nix/store/c3nr4r9ld8np3yz31circxfqipinfchq-qemu-5.2.0/libexec/../etc/qemu/bridge.conf'

Calling qemu-bridge-helper shows the same error:

❯ qemu-bridge-helper --use-vnet --br=virbr0 --fd=31                                                          YOLOv5 
failed to parse default acl file `/nix/store/c3nr4r9ld8np3yz31circxfqipinfchq-qemu-5.2.0/libexec/../etc/qemu/bridge.conf'

It's trying to read a file that not exists /nix/store/c3nr4r9ld8np3yz31circxfqipinfchq-qemu-5.2.0/libexec/../etc/qemu/bridge.conf

Changing VM to User mode the VM starts.

To Reproduce
Steps to reproduce the behavior:

1. Start a libvirt VM
2. It fails to start

Expected behavior

VM starts.

Screenshots
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Additional context
Add any other context about the problem here.

Notify maintainers

@drewrisinger @globin @fpletz

Metadata
Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

• system: "x86_64-linux"
• host os: Linux 5.10.11, NixOS, 21.03.20210201.65211f5 (Okapi)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.4pre20201205_a5d85d0
• channels(root): "nixos-21.03pre250162.34ad166a830"
• nixpkgs: /etc/nixpkgs

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module: qemu or libvirt