hw/i386/xen/xen-hvm: Inline xen_piix_pci_write_config_client() and remove it

hw/i386/xen/xen-hvm: Inline xen_piix_pci_write_config_client() and remove it

xen_piix_pci_write_config_client() is implemented in the xen sub tree and
uses PIIX constants internally, thus creating a direct dependency on
PIIX. Now that xen_set_pci_link_route() is stubbable, the logic of
xen_piix_pci_write_config_client() can be moved to PIIX which resolves
the dependency.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Message-Id: <20220626094656.15673-3-shentey@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>