virtio: introduce virtio_get_num_queues()

This patch introduces virtio_get_num_queues() which iterates the vqs
array and return the number of virtqueues used by device.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>