Add XENBUS_SHARED_INFO method to check whether event upcalls are supported

Add XENBUS_SHARED_INFO method to check whether event upcalls are supported

Upcalls can only be supported on a vCPU that has a vcpu_info structure
available to Xen; that is one with a vcpu_id < XEN_LEGACY_MAX_VCPUS (32) or
one that has explicitly registered a vcpu_info. This is determined during
SharedInfoAcquire() but the information needs to be avalable to the code in
evtchn.c. Hence this patch adds a new 'UpcallSupported' method to
XENBUS_SHARED_INFO (consequently bumping the interface version to 3).

Signed-off-by: Paul Durrant <pdurrant@amazon.com>