As the NBD server spawned by qemu can only listen on a single socket,
we're dependent on a version being passed to vm_start that indicates
which protocol can be used, TCP or Unix, by the source node.
The change in socket type (TCP to Unix) comes with a different URI. For
unix sockets it has the form: 'nbd:unix:<path/to/socket>:exportname=<device>'.
Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>