tools/dombuilder: Remove p2m_guest from the common interface

tools/dombuilder: Remove p2m_guest from the common interface

In-guest p2m's are a concept specific to x86 PV guests.  alloc_p2m_list() is
the only hook which initialises dom->p2m_guest, making
xc_dom_update_guest_p2m() a nop for non-PV guests.

Move p2m_guest into xc_dom_image_x86 and adjust alloc_p2m_list() to match.

Drop xc_dom_update_guest_p2m() entirely.

One caller, move_l3_below_4G(), only uses it to modify a single entry, so
rewriting the whole guest p2m is wasteful - opencode the single update
instead.  The other caller is common code.  Instead, move the logic into the
setup_pgtables() hooks, which know their own sizeof_pfn and can do away with
the switch statement.

No change in the constructed guests.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wl@xen.org>