virtio_pci: extract the logic of active vq for modern pci

Introduce vp_active_vq() to configure vring to backend after vq attach
vring. And configure vq vector if necessary.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20220801063902.129329-32-xuanzhuo@linux.alibaba.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>