Support standard NBD URL

[nirs]

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

NBD URI	Export name
nbd://example.com/disk	disk
nbd+unix:///disk?socket=sock	disk
nbd://example.com/	(empty string)
nbd://example.com	(empty string)
nbd://example.com//disk	/disk
nbd://example.com/hello%20world	hello world

imageio supports qemu non-standard syntax, and the nbd://
URL syntax.

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

We need to support the new standard syntax for better compatibility with
virt-v2v and other tools using modern NBD URLs.

NBD URL are generated by vdsm when starting NBD server or backup. Vdsm
can use the new URL syntax only when all hosts are running imageio version
that understand the new syntax.

Plan:

• Add support for new NBD URL
• Keep support for old URI if possible

See also oVirt/vdsm#174.

Original bug: https://bugzilla.redhat.com/1849091

[rwmjones]

FYI libnbd can parse URIs: https://libguestfs.org/nbd_connect_uri.3.html

[nirs]

We don't use libnbd yet, and we cannot use it since it does not support the old non-standard URL generated by vdsm. After we switch to standard URLs and we ensure that we never run on a host with older vdsm generating non standard NBD URLs, we can switch.

Opened #69 for switching to libnbd.