add vm network packet drop and packet error statistics

add vm statistics:
network.incoming.dropped
    The number of discarded packets received by the port
network.incoming.errors
    The number of error packets received by the port
network.outgoing.dropped
    The number of discarded packets transfered by the port
network.outgoing.errors
    The number of error packets transfered by the port

the data can be got from libvirt:
[root@TECS-27 ~]# virsh domifstat 8 tap5150a1f2-9a
tap5150a1f2-9a rx_bytes 718
tap5150a1f2-9a rx_packets 9
tap5150a1f2-9a rx_errs 0 #network.incoming.errors
tap5150a1f2-9a rx_drop 0 #network.incoming.dropped
tap5150a1f2-9a tx_bytes 1864890
tap5150a1f2-9a tx_packets 5459
tap5150a1f2-9a tx_errs 0 #network.outgoing.errors
tap5150a1f2-9a tx_drop 0 #network.outgoing.dropped