vm_event: make sure the domain is paused in key domctls

vm_event: make sure the domain is paused in key domctls

This patch pauses the domain for all writes through the 'ad'
pointer in monitor_domctl(), defers a domain_unpause() call until
after the CRs are updated for the MONITOR_EVENT_WRITE_CTRLREG
case, and makes sure that the domain is paused for both vm_event
enable and disable cases in vm_event_domctl().
Thanks go to Andrew Cooper for his review and suggestions.

Signed-off-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>