x86/mm: move guest_physmap_{add,remove}_page()

x86/mm: move guest_physmap_{add,remove}_page()

... to a new file, separating the functions from their HVM-specific
backing ones, themselves only dealing with the non-translated case.

To avoid having a new CONFIG_HVM conditional in there, do away with
the inline placeholder.

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