xenmem_add_to_physmap_one() has no need to know of XENMAPSPACE_gmfn_range

xenmem_add_to_physmap_one() has no need to know of XENMAPSPACE_gmfn_range

As its name says, it handles a single GMFN only anyway. Note that ARM
needs no adjustment, as it doesn't handle the two types at all.

Also take the opportunity and clean up the handling of XENMAPSPACE_gmfn
a little: There's no point in going through "idx" when capturing the MFN.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>