common/vm_event: Initialize vm_event lists on domain creation

common/vm_event: Initialize vm_event lists on domain creation

The patch splits the vm_event into three structures:vm_event_share,
vm_event_paging, vm_event_monitor. The allocation for the
structure is moved to vm_event_enable so that it can be
allocated/init when needed and freed in vm_event_disable.

Signed-off-by: Alexandru Isaila <aisaila@bitdefender.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>