vz: keep subscription to performance events thru domain lifetime

vz: keep subscription to performance events thru domain lifetime

The approach of subscribing on first stat API call and then waiting
for receiving of performance event from sdk to process the call originates
in times when every vz libvirt connections spawns its own sdk connection.
Thus without this waiting virsh stat call would return empty stats. Now
with single sdk connection this scheme is unnecessary complicated.

This patch subscribes to performance events on first domain appearence
and unsubscribe on its removing.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>
Signed-off-by: Maxim Nestratov <mnestratov@virtuozzo.com>