x86: clean up vm_event-related code in asm-x86/domain.h

x86: clean up vm_event-related code in asm-x86/domain.h

As suggested by Jan Beulich, moved struct monitor_write_data from
struct arch_domain to struct arch_vcpu, as well as moving all
vm_event-related data from asm-x86/domain.h to struct vm_event,
and allocating it dynamically only when needed.

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