vdpa_sim: implement get_iova_range()

This implements a sample get_iova_range() for the simulator which
advertise [0, ULLONG_MAX] as the valid range.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20201023090043.14430-4-jasowang@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>