virtio-pci-modern: introduce vp_modern_generation()

This patch introduces vp_modern_generation() to get device generation.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210104065503.199631-9-jasowang@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>