virtio: clear signalled_used_valid when switching from dataplane

virtio: clear signalled_used_valid when switching from dataplane

When the dataplane thread stops, its vring.c implementation synchronizes
vring state back to virtio.c so we can continue emulating the virtio
device.

This patch ensures that virtio.c's signalled_used_valid flag is reset so
that we do not suppress guest notifications due to stale signalled_used
values.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 6793dfd1b6a99a79b9f2e3c4d6625ccd6513f240)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>