net/colo-compare.c: Adjust net queue pop order for performance

The packet_enqueue() use g_queue_push_tail() to
enqueue net packet, so it is more efficent way use
g_queue_pop_head() to get packet for compare.
That will improve the success rate of comparison.
In my test the performance of ftp put 1000M file
will increase 10%

Signed-off-by: Zhang Chen <zhangchen.fnst@cn.fujitsu.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>