virtio-pci-modern: introduce helper to set config vector

This patch introduces vp_modern_config_vector() for setting config
vector.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210104065503.199631-6-jasowang@redhat.com


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