qemu couldn't connect the unix domain socket

[Li Feng <fengli@smartx.com>]

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Hi,
I just tested the master code, the passt just exited without any error.

```
$ ./passt
Outbound interface: ens192
ARP:
    address: 00:50:56:be:9d:1f
DHCP:
    assign: 192.168.64.217
    mask: 255.255.240.0
    router: 192.168.64.1
        search:
            .
UNIX domain socket bound at /tmp/passt_1.socket

You can now start qrap:
    ./qrap 5 kvm ... -net socket,fd=5 -net nic,model=virtio
or directly qemu, patched with:
    qemu/0001-net-Allow-also-UNIX-domain-sockets-to-be-used-as-net.patch
as follows:
    kvm ... -net socket,connect=/tmp/passt_1.socket -net nic,model=virtio
21-10-26 13:23:43 root(a)192.168.64.217:~/passt(master✗)
```
Another terminal:
```
$ bash x.sh
recv: Connection reset by peer
Probe of /tmp/passt_1.socket failed
connect: No such file or directory
Probe of /tmp/passt_2.socket failed
connect: No such file or directory
Probe of /tmp/passt_3.socket failed
...
```

```
$ cat x.sh
./qrap 5 /root/qemu-master/build/qemu-system-x86_64 -uuid
1869b108-42b3-42a7-852e-70261d73f6a9 -name
guest=1869b108-42b3-42a7-852e-70261d73f6a9 -cpu host -enable-kvm -smp
4 -device pci-bridge,chassis_nr=1,id=pci.0 -device
pci-bridge,chassis_nr=1,id=pci.1 -device
virtio-scsi-pci,id=scsi0,bus=pci.0,addr=0x10 -fsdev
local,security_model=mapped,id=fsdev-fs0,path=/root -device
virtio-9p-pci,id=fs0,fsdev=fsdev-fs0,mount_tag=fs0 -device
intel-iommu,device-iotlb=on,intremap=on -M
q35,accel=kvm,kernel-irqchip=split -serial mon:stdio -nographic -m
size=2048M,maxmem=32G,slots=128 -device virtio-balloon -drive
file=/root/fedora34.qcow2,format=qcow2,cache=none,aio=native,if=none,id=drive-virtio-disk1,file.locking=off
-device virtio-blk-pci,scsi=off,drive=drive-virtio-disk1,id=virtio-disk1,bootindex=1
-vnc 0.0.0.0:101 -chardev
socket,id=charmonitor,path=/var/lib/libvirt/qemu/domain-222-29ec19c9-d330-4949-b/monitor.sock,server,nowait
-mon chardev=charmonitor,id=monitor,mode=control -qmp
tcp:0.0.0.0:2235,server,nowait -device
virtio-serial-pci,id=virtio-serial0,max_ports=16 -net socket,fd=5 -net
nic,model=virtio
```

Thanks,
Feng Li