qemu: support URI syntax for NBD

qemu: support URI syntax for NBD

QEMU 1.3 and newer support an alternative URI-based syntax to specify
the location of an NBD server.  Libvirt can keep on using the old
syntax in general, but only the URI syntax supports IPv6 addresses.

The URI syntax also supports relative paths to Unix sockets.  These
should never be used but aren't explicitly blocked either by the parser,
so support it just in case.

The URI syntax is intentionally compatible with Gluster's, and the
code can be reused.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>