x86/vm_event: reset monitor in vm_event_cleanup_domain()

x86/vm_event: reset monitor in vm_event_cleanup_domain()

It is currently possible to leave a monitor flag enabled even
after vm_event_cleanup_domain() has been called, potentially
leading to a crash in hvm_msr_write_intercept() and hvm_set_crX()
(when v->arch.vm_event has become NULL, but the corresponding
corresponding v->domain->arch.monitor flag is non-zero).
This patch zeroes out arch.monitor in vm_event_cleanup_domain().

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