Description of problem:

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 bug 1849091 - 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.