hw/xen: Hook up emulated implementation for event channel operations

hw/xen: Hook up emulated implementation for event channel operations

We provided the backend-facing evtchn functions very early on as part of
the core Xen platform support, since things like timers and xenstore need
to use them.

By what may or may not be an astonishing coincidence, those functions
just *happen* all to have exactly the right function prototypes to slot
into the evtchn_backend_ops table and be called by the PV backends.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>