x86: make pgd/pud/pmd/pte_none consistent

The _none test is done differently for every level of the pagetable.
Standardize them by:

 1: Use the native_X_val to extract the raw entry, with no need to go
    via paravirt_ops, diff -r 1d0646d0d319 arch/x86/include/asm/pgtable.h, and
 2: Compare with 0 rather than using a boolean !, since they are actually values
    and not booleans.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>