x86/pv: Clean up cr3 handling in arch_set_info_guest()

x86/pv: Clean up cr3 handling in arch_set_info_guest()

All of this code lives inside CONFIG_PV which means gfn == mfn, and the
fill_ro_mpt() calls clearly show that the value is used untranslated.

Change cr3_gfn to a suitably typed cr3_mfn, and replace get_page_from_gfn()
with get_page_from_mfn().

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>