xen/pt: split out calculation of throughable mask in PCI config space handling

xen/pt: split out calculation of throughable mask in PCI config space handling

This is just to avoid having to adjust that calculation later in
multiple places.

Note that including ->ro_mask in get_throughable_mask()'s calculation
is only an apparent (i.e. benign) behavioral change: For r/o fields it
doesn't matter > whether they get passed through - either the same flag
is also set in emu_mask (then there's no change at all) or the field is
r/o in hardware (and hence a write won't change it anyway).

This is a preparatory patch for XSA-131.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>