virttest.qemu_vm: modify nic hotplug fd

In autotest python get the fd by os.open("dev/net/tun"),
then using getfd pass the fd to the qemu process. some
times the fd number qemu get is not same as parent's fd.
will call nic hotplug error, this patch fix this issue.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>

Conflicts:
	virttest/qemu_vm.py