hw/i386/xen/xen-hvm: Allow for stubbing xen_set_pci_link_route()

The only user of xen_set_pci_link_route() is
xen_piix_pci_write_config_client() which implements PIIX-specific logic in
the xen namespace. This makes xen-hvm depend on PIIX which could be
avoided if xen_piix_pci_write_config_client() was implemented in PIIX. In
order to do this, xen_set_pci_link_route() needs to be stubbable which
this patch addresses.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Message-Id: <20220626094656.15673-2-shentey@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>