Some guests read back queue size after writing it.
Always update the on size write otherwise they might be confused.

Cc: qemu-stable@nongnu.org
Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
Message-Id: <20191224081446.17003-1-dplotnikov@virtuozzo.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>