hw/xen: Add backend implementation of interdomain event channel support

hw/xen: Add backend implementation of interdomain event channel support

The provides the QEMU side of interdomain event channels, allowing events
to be sent to/from the guest.

The API mirrors libxenevtchn, and in time both this and the real Xen one
will be available through ops structures so that the PV backend drivers
can use the correct one as appropriate.

For now, this implementation can be used directly by our XenStore which
will be for emulated mode only.

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