qemu: respond to NETDEV_VHOST_USER_DISCONNECTED event

qemu: respond to NETDEV_VHOST_USER_DISCONNECTED event

This response to this event is identical to NETDEV_STREAM_DISCONNECTED
(start a new passt process to replace the one that just disappeared -
see commitf62ce81b8a5), except that the new passt process will have
"--vhost-user" appended to the commandline. Fortunately that
difference is already handled based on the virDomainNetDef contents,
so we can, in fact, respond to the new event in exactly the same
manner.

Signed-off-by: Laine Stump <laine@redhat.com>
Tested-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>