x86: Fix guest_physmap_add_entry checks

x86: Fix guest_physmap_add_entry checks

guest_physmap_add_entry() checks to see if the given mfn and gpfn
range in the p2m and m2p tables is already mapped before overwriting
the maps, and attempts to do something reasonable so that we don't
have any "dangling" pointers.

Unfortunately, these checks got broken when the page_order argument
was added.  Each individual p2m and m2p entry needs to be checked, not
just the first page in a page order.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>