x86/mm: subsume set_gpfn_from_mfn() into guest_physmap_add_page()

x86/mm: subsume set_gpfn_from_mfn() into guest_physmap_add_page()

The two callers in common/memory.c currently call set_gpfn_from_mfn()
themselves, so moving the call into guest_physmap_add_page() helps
tidy their code.

The two callers in common/grant_table.c fail to make that call alongside
the one to guest_physmap_add_page(), so will actually get fixed by the
change.

Other (x86) callers are HVM only and are hence unaffected by a change
to the function's !paging_mode_translate() part.

Sadly this isn't enough yet to drop Arm's dummy macro, as there's one
more use in page_alloc.c.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>