qga: add network stats to guest-network-get-interfaces

we can get the network interface statistics inside a virtual machine by
guest-network-get-interfaces command. it is very useful for us tomonitor
and analyze network traffic.

Signed-off-by: ZhiPeng Lu <lu.zhipeng@zte.com.cn>
* don't rely on sizeof(wchar[]) for wchar[] indexing
* avoid camelCase variable names
* fix up getline() usage
* condensed commit subject line
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>