vm_event: fix XEN_VM_EVENT_RESUME domctl

vm_event: fix XEN_VM_EVENT_RESUME domctl

Make XEN_VM_EVENT_RESUME return 0 in case of success, instead of
-EINVAL.
Remove vm_event_resume form vm_event.h header and set the function's
visibility to static as is used only in vm_event.c.
Move the vm_event_check_ring test inside vm_event_resume in order to
simplify the code.

Signed-off-by: Petre Pircalabu <ppircalabu@bitdefender.com>
Acked-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
master commit: b32c0446b103aa801ee18780b2fdd78dfc0b9052
master date: 2019-04-05 15:42:03 +0200