vz: add net dev statistiscs
    
    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: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>