x86/pagewalk: Consistently use guest_walk_*() helpers for translation

x86/pagewalk: Consistently use guest_walk_*() helpers for translation

hap_p2m_ga_to_gfn() and sh_page_fault() currently use guest_l1e_get_gfn() to
obtain the translation of a pagewalk.  This is conceptually wrong (the
semantics of gw.l1e is an internal detail), and will actually be wrong when
PSE36 superpage support is fixed.  Switch them to using guest_walk_to_gfn().

guest_walk_tables() also uses guest_l1e_get_gfn(), and is updated for
consistency.

Take the opportunity to const-correct the walk_t parameter of the
guest_walk_to_*() helpers, and implement guest_walk_to_gpa() in terms of
guest_walk_to_gfn() to avoid duplicating the actual translation calculation.

While editing guest_walk_to_gpa(), fix a latent bug by causing it to return
INVALID_PADDR rather than 0 for a failed translation, as 0 is also a valid
successful result.  The sole caller, sh_page_fault(), has already confirmed
that the translation is valid, so this doesn't cause a behavioural change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Acked-by: George Dunlap <george.dunlap@citrix.com>