vhost: fix interrupt mitigation with raw sockets

A thinko in code means we never trigger interrupt
mitigation. Fix this.

Reported-by: Juan Quintela <quintela@redhat.com>
Reported-by: Unai Uribarri <unai.uribarri@optenet.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>