xen/arm: p2m: Limit call to mem access code use in get_page_from_gva

xen/arm: p2m: Limit call to mem access code use in get_page_from_gva

Mem access has only an impact on the hardware translation between a
guest virtual address and the machine physical address. So it is not
necessary to fallback to memaccess for all the other case (e.g when it
is not possible to acquire the page behind the MFN).

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by Razvan Cojocaru <rcojocaru@bitdefender.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>