ifcvf: implement vdpa_config_ops.get_vq_irq()

This commit implemented vdpa_config_ops.get_vq_irq() in ifcvf,
and initialized vq irq to -EINVAL. So that ifcvf can report
irq number of a vq, or -EINVAL if the vq is not assigned an
irq number.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Suggested-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20200731065533.4144-6-lingshan.zhu@intel.com


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