vm_event: clean up control-register-write vm_events and add XCR0 event

vm_event: clean up control-register-write vm_events and add XCR0 event

As suggested by Andrew Cooper, this patch attempts to remove
some redundancy and allow for an easier time when adding vm_events
for new control registers in the future, by having a single
VM_EVENT_REASON_WRITE_CTRLREG vm_event type, meant to serve CR0,
CR3, CR4 and (newly introduced) XCR0. The actual control register
will be deduced by the new .index field in vm_event_write_ctrlreg
(renamed from vm_event_mov_to_cr).

Signed-off-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>