virsh: Display vhostuser socket path in domblklist

virsh: Display vhostuser socket path in domblklist

The domblklist command is designed to show a brief information
about the blocks of a domain. One piece of information that is
shows is "Target "and "Source". Before the modification, the
Vhost disk of SPDK is displayed as "-". After the modification,
the socket associated with it can be displayed.

Signed-off-by: dinglimin <dinglimin@cmss.chinamobile.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>