tests/test-filter-mirror:add filter-mirror unit test

tests/test-filter-mirror:add filter-mirror unit test

In this unit test we will test the mirror function.

start qemu with:
      -netdev socket,id=qtest-bn0,fd=sockfd
      -device e1000,netdev=qtest-bn0,id=qtest-e0
      -chardev socket,id=mirror0,path=/tmp/filter-mirror-test.sock,server,nowait
      -object filter-mirror,id=qtest-f0,netdev=qtest-bn0,queue=tx,outdev=mirror0

We inject packet to netdev socket id = qtest-bn0,
filter-mirror will copy and mirror the packet to mirror0.
we read packet from mirror0 and then compare to what we injected.

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