mem_event: Add support for MEM_EVENT_REASON_MSR

mem_event: Add support for MEM_EVENT_REASON_MSR

Add the new MEM_EVENT_REASON_MSR event type. Works similarly
to the other register events, except event.gla always contains
the MSR address (in addition to event.gfn, which holds the value).
MEM_EVENT_REASON_MSR does not honour the HVMPME_onchangeonly bit,
as doing so would complicate the hvm_msr_write_intercept()
switch-based handling of writes for different MSR addresses,
with little added benefit.

Signed-off-by: Razvan Cojocaru <rzvncj@gmail.com>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>