tools/libxc: rename pfn_to_mfn to xc_pfn_to_mfn

tools/libxc: rename pfn_to_mfn to xc_pfn_to_mfn

Also refactor the contents of xc_pfn_to_mfn().  It is functionally identical,
but contains less lisp, fewer magic numbers, and more description of why 32bit
guests are treated differently.

Note that this does not affect pfn_to_mfn() in xc_domain_save.c  That was
already a macro which aliased pfn_to_mfn() in xg_private.h but without
actually using it.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>