virtio-pci-modern: factor out modern device initialization logic

This patch factors out the modern device initialization logic into a
helper. Note that it still depends on the caller to enable pci device
which allows the caller to use e.g devres.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210104065503.199631-4-jasowang@redhat.com


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