xen/grant: Switch {create, replace}_grant_p2m_mapping to typesafe MFN

xen/grant: Switch {create, replace}_grant_p2m_mapping to typesafe MFN

The current prototype is slightly confusing because it takes a guest
physical address and a machine physical frame (not address!). Switching to
MFN will improve safety and reduce the chance to mistakenly invert the
2 parameters.

Signed-off-by: Julien grall <julien.grall@arm.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>