xen/x86: Use mfn_to_gfn rather than mfn_to_gmfn

xen/x86: Use mfn_to_gfn rather than mfn_to_gmfn

mfn_to_gfn and mfn_to_gmfn are doing exactly the same except the former
is using mfn_t and gfn_t (return type).

Furthermore, the naming of the former is more consistent with the
current naming scheme (GFN/MFN). So replace mfn_to_gmfn with
mfn_to_gfn in x86 code.

Take the opportunity to convert some of the callers to use typesafe GFN and
format the message correctly.

No functional changes.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
--
    Changes in v3:
        - The hunk in x86/mm.c is not necessary anymore
        - Update printk message to use GFN rather than frame when suitable
        - Update commit message with some NITs
        - Add Jan's reviewed-by

    Changes in v2:
        - mfn_to_gfn now returns a gfn_t
        - Use %pd and PRI_gfn when possible in the message
        - Don't split format string to help grep/ack.