vp_vdpa: add vq irq offloading support

This patch implements the get_vq_irq() callback for virtio pci devices
to allow irq offloading.

Signed-off-by: Wu Zongyong <wuzongyong@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/bb091e5505db704dd620f8854a7aebc921d2a752.1635493219.git.wuzongyong@linux.alibaba.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>