agent-ctl: Fix abstract socket connections

Unbreak the `agent-ctl` tool connecting to the agent with a Unix domain
socket.

It appears that [1] changed the behaviour of connecting to the agent
using a local Unix socket (which is not used by Kata under normal
operation).

The change can be seen by reverting to commit
72b8144b56 (the one before [1]) and
running the agent manually as:

```bash
$ sudo KATA_AGENT_SERVER_ADDR=unix:///tmp/foo.socket target/x86_64-unknown-linux-musl/release/kata-agent
```

Before [1], in another terminal we see this:

```bash
$ sudo lsof -U 2>/dev/null |grep foo|awk '{print $9}'
@/tmp/foo.socket@
```

But now, we see the following:

```bash
$ sudo lsof -U 2>/dev/null |grep foo|awk '{print $9}'
@/tmp/foo.socket
```

Note the last byte which represents a nul (`\0`) value.

The `agent-ctl` tool used to add that trailing nul but now it seems to not
be needed, so this change removes it, restoring functionality. No
external changes are necessary so the `agent-ctl` tool can connect to
the agent as below like this:

```bash
$ cargo run -- -l debug connect --server-address "unix://@/tmp/foo.socket" --bundle-dir "$bundle_dir" -c Check -c GetGuestDetails
```

[1] - https://github.com/kata-containers/kata-containers/issues/3124

Fixes: #4164.

Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>

src/tools/agent-ctl/src/client.rs

@@ -473,10 +473,8 @@ fn create_ttrpc_client(
             if path.starts_with('@') {
                 abstract_socket = true;
 
-                // Remove the magic abstract-socket request character ('@')
-                // and crucially add a trailing nul terminator (required to
-                // interoperate with the ttrpc crate).
-                path = path[1..].to_string() + &"\x00".to_string();
+                // Remove the magic abstract-socket request character ('@').
+                path = path[1..].to_string();
             }
 
             if abstract_socket {