xen/common: grant-table: only call IOMMU if paging mode translate is disabled

xen/common: grant-table: only call IOMMU if paging mode translate is disabled

From Xen point of view, ARM guests are PV guest with paging auto translate
enabled.

When IOMMU support will be added for ARM, mapping grant ref will always crash
Xen due to the BUG_ON in __gnttab_map_grant_ref.

On x86:
    - PV guests always have paging mode translate disabled
    - PVH and HVM guests have always paging mode translate enabled

It means that we can safely replace the check that the domain is a PV guests
by checking if the guest has paging mode translate enabled.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Keir Fraser <keir@xen.org>