x86: synchronize PCI config space access decoding

x86: synchronize PCI config space access decoding

Both PV and HVM logic have similar but not similar enough code here.
Synchronize the two so that
- in the HVM case we don't unconditionally try to access extended
  config space
- in the PV case we pass a correct range to the XSM hook
- in the PV case we don't needlessly deny access when the operation
  isn't really on PCI config space
All this along with sharing the macros HVM already had here.

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