x86/pagewalk: Helpers for reserved bit handling

x86/pagewalk: Helpers for reserved bit handling

Some bits are unconditionally reserved in pagetable entries, or reserved
because of alignment restrictions.  Other bits are reserved because of control
register configuration.

Introduce helpers which take an individual vcpu and guest pagetable entry, and
calculates whether any reserved bits are set.

While here, add a couple of newlines to aid readability.

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