vm-event: MAINTAINERS fix

vm-event: MAINTAINERS fix

Fix vm-event section of MAINTAINERS file: add back files x86/hvm/monitor.c,
asm-x86/hvm/monitor.h, x86/monitor.c, x86/vm_event.c and sort entries
alphabetically.

Culprits which got MAINTAINERS out-of-sync:
c/s ca63cee: "monitor: Rename hvm/event to hvm/monitor":
    - added x86/hvm/monitor.c & asm-x86/hvm/monitor.h w/o MAINTAINERS update
c/s ec89da2: "MAINTAINERS: update monitor/vm_event covered code":
    - (mistakenly?) removed both x86/monitor.c & x86/vm_event.c

Signed-off-by: Corneliu ZUZU <czuzu@bitdefender.com>
Acked-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>