vdpa: introduce config op to get valid iova range

This patch introduce a config op to get valid iova range from the vDPA
device.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20201023090043.14430-2-jasowang@redhat.com


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