vDPA: add get_vq_irq() in vdpa_config_ops

This commit adds a new function get_vq_irq() in struct
vdpa_config_ops, which will return the irq number of a
virtqueue.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Suggested-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20200731065533.4144-4-lingshan.zhu@intel.com


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