virtio-pci-modern: introduce vp_modern_set_queue_vector()

This patch introduces a helper to set virtqueue MSI vector.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210104065503.199631-10-jasowang@redhat.com


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