vdpa: Use simpler version of ida allocation

vdpa doesn't have any specific need to define start and end range of the
device index.
Hence use the simper version of the ida allocator.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Eli Cohen <elic@nvidia.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20201112064005.349268-3-parav@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>