virtio-pci-modern: introduce helper for getting queue nums

This patch introduces helper for getting queue num of modern device.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210104065503.199631-14-jasowang@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>