Xen events ABI compatiblity

[Wenzel]

The Xen events ABI has changed a lot along the Xen releases.

Right now, we are following the ABI from Xen 4.11 and onwards. If we want to be compatible with all release, we need to follow what LibVMI implemented with xen_events_abi.h

And deduce the vm_event version from the Xen version: https://github.com/libvmi/libvmi/blob/master/libvmi/driver/xen/xen_events.c#L3213

[Wenzel]

Related: https://github.com/libvmi/libvmi/blob/master/libvmi/driver/xen/xen_events.c#L3213