agent: fix wrong regular exp to fetch guest-cid
Fix the incorrect regular expression to fetch the guest context ID.
In " [^,][^,]* ", [^,]* will match to the next ",",
which is after "socket",  so finally got incorrect result.
Use egrep -o "guest-cid=[0-9]*" instead.
Fixes:
@@ -80,7 +80,7 @@ $ sudo docker export $(sudo docker create "$image") | tar -C "$rootfs_dir" -xvf
value:
```sh
- $ guest_cid=$(ps -ef | grep qemu-system-x86_64 | egrep -o "guest-cid=[^,][^,]*" | cut -d= -f2)
+ $ guest_cid=$(ps -ef | grep qemu-system-x86_64 | egrep -o "guest-cid=[0-9]*" | cut -d= -f2)
```
1. Run the tool to connect to the agent:
