libvhost-user: support resuming vq->last_avail_idx based on used_idx

This is the same workaround as commit 523b018d, which was lost
with libvhost-user transition in commit e10e798c.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 35480cbf)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>