hw/xen: add get_frontend_path() method to XenDeviceClass

hw/xen: add get_frontend_path() method to XenDeviceClass

The primary Xen console is special. The guest's side is set up for it by
the toolstack automatically and not by the standard PV init sequence.

Accordingly, its *frontend* doesn't appear in …/device/console/0 either;
instead it appears under …/console in the guest's XenStore node.

To allow the Xen console driver to override the frontend path for the
primary console, add a method to the XenDeviceClass which can be used
instead of the standard xen_device_get_frontend_path()

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>