virtio-mmio: update queue size on guest write

virtio-mmio: update queue size on guest write

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>
(cherry picked from commit 1049f4c62c4070618cc5defc9963c6a17ae7a5ae)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>