x86/p2m: pass old PTE directly to write_p2m_entry_pre() hook

x86/p2m: pass old PTE directly to write_p2m_entry_pre() hook

In no case is a pointer to non-const needed. Since no pointer arithmetic
is done by the sole user of the hook, passing in the PTE itself is quite
fine.

While doing this adjustment also
- drop the intermediate sh_write_p2m_entry_pre():
  sh_unshadow_for_p2m_change() can itself be used as the hook function,
  moving the conditional into there,
- introduce a local variable holding the flags of the old entry.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>