x86: simplify is_guest_l2_slot()

x86: simplify is_guest_l2_slot()

is_pv_32bit_domain() has become expensive, and its use here is
redundant: Only 32-bit guests would ever get PGT_pae_xen_l2 set on
their L2 page table pages anyway. (If some other error does lead to
PGT_pae_xen_l2 ending up anywhere else, we still don't want to allow a
guest to control the entries.)

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>