virtio_balloon: separate vm events into a function

All the VM events related statistics have dependence on
'CONFIG_VM_EVENT_COUNTERS', separate these events into a function to
make code clean. Then we can remove 'CONFIG_VM_EVENT_COUNTERS' from
'update_balloon_stats'.

Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
Message-Id: <20240423034109.1552866-2-pizhenwei@bytedance.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>