Support standard NBD URL syntax

[ahadas]

Description

NBD protocol introduced new URL syntax recenlty:
https://github.com/NetworkBlockDevice/nbd/blob/master/doc/uri.md

NBD URI	Export name
nbd+unix:///disk?socket=sock	disk
nbd+unix:///?socket=sock	(empty string)
nbd://example.com/disk	disk
nbd://example.com/	(empty string)

Vdsm generates qemu non-standard syntax:

QEMU NBD URI	Export name
nbd:unix:/socket:exportname=disk	disk
nbd:example.com/disk	disk

We want to switch to the standard NBD URL syntax for better compatibility with
virt-v2v, and possibly allowing backup using NBD socket.

Vdsm generates NBD URL in:

• NBD.start_server
• VM.start_backup

In both cases we need to switch to the new syntax.

This change depends on oVirt/ovirt-imageio#68
we cannot generate URLs that imageio does not support, since imageio is
consuming these URLs when downloading disks and backups.

We also need to check that qemu-nbd and qemu support the new syntax, we never
tested the new syntax.

How this can work with mixed environment?

Old hosts will generate the old non-standard NBD URL, and new hosts the standard NBD URL.
Upload on new host uses imageio supporting new NBD URL. Upload on old hosts use imageio
that may not support new standard NBD URL.

Plan

• Require imageio version supporting standard NBD URL
• Generate standard NBD URL

More info

Original bug: https://bugzilla.redhat.com/1849097