qga: replace GetIfEntry with GetIfEntry2 for interface stats

The data obtained by GetIfEntry is 32 bits, and it may overflow. Thus
using GetIfEntry2 instead of GetIfEntry.

Signed-off-by: ZhiPeng Lu <lu.zhipeng@zte.com.cn>
*avoid CamelCase variable names
*update field names for MIB_IFROW -> MIB_IF_ROW2
*dynamically probe for GetIfIndex2 to deal with older OSs
*check return value from get_interface_index
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
