xen/pt: unknown PCI config space fields should be read-only
... by default. Add a per-device "permissive" mode similar to pciback's
to allow restoring previous behavior (and hence break security again,
i.e. should be used only for trusted guests).
This is part of 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>)