libvhost-user: quit when no more data received

End processing of messages when VHOST_USER_NONE
is received.

Without this we run into a vubr_panic() call and get
"PANIC: Unhandled request: 0"

Signed-off-by: Jens Freimann <jfreiman@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
