x86/P2M: move map_domain_gfn() (again)

x86/P2M: move map_domain_gfn() (again)

The main user is the guest walking code, so move it back there; commit
9a6787cc3809 ("x86/mm: build map_domain_gfn() just once") would perhaps
better have kept it there in the first place. This way it'll only get
built when it's actually needed (and still only once).

This also eliminates one more CONFIG_HVM conditional from p2m.c.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>