xen-pciback: also support disabling of bus-mastering and memory-write-invalidate

It's not clear to me why only the enabling operation got handled so
far.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>