xen/pt: mark reserved bits in PCI config space fields

The adjustments are solely to make the subsequent patches work right
(and hence make the patch set consistent), namely if permissive mode
(introduced by the last patch) gets used (as both reserved registers
and reserved fields must be similarly protected from guest access in
default mode, but the guest should be allowed access to them in
permissive mode).

This is a preparatory patch for XSA-131.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
