xen/pci: support multi-segment systems

Now that the hypercall interface changes are in -unstable, make the
kernel side code not ignore the segment (aka domain) number anymore
(which results in pretty odd behavior on such systems). Rather, if
only the old interfaces are available, don't call them for devices on
non-zero segments at all.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
[v1: Edited git description]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>