Populate counters SDK currenly supports:
 rx_bytes
 rx_packets
 tx_bytes
 tx_packets

Comments.

Use vzDomObjFromDomainRef/virDomainObjEndAPI pair to get domain
object as we use prlsdkGetStatsParam that can release domain
object lock and thus we need a reference in case domain
is deleated meanwhile.
Signed-off-by: NNikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>