hw/pci-bridge: expose _test parameter in SHPC_VMSTATE()

Change the signature of the function-like macro SHPC_VMSTATE(), so that we
can produce and expect this field conditionally in the migration stream,
starting with an upcoming patch.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
