The UDP port forwarder previously used a single gRPC stream for all
clients, which could cause responses from the guest to be sent to the
wrong client on the host.

This occurred because the stream was created before client connections
were demultiplexed by `gvisor-tap-vsock`'s `UDPProxy`.

The root cause is the interaction with `gvisor-tap-vsock`'s `UDPProxy`,
which handles client demultiplexing internally based on the source
address of incoming datagrams. It expects its `dialer` function to
return a new `net.Conn` for each new client it detects.

This commit moves the gRPC stream creation into the `UDPProxy` dialer
function. This ensures a new, dedicated stream is created for each new
client, fixing the incorrect response routing.

Signed-off-by: Viktor Oreshkin <imselfish@stek29.rocks>
(cherry picked from commit a04f244)
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

Fix issue 3720

Thanks to pavelanni for providing the fix generated by Claude

Co-authored-by: Oleksandr Redko <oleksandr.red+github@gmail.com>
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
(cherry picked from commit 2b8bb39)
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

```
Error: pkg\instance\start.go:274:65: SA4023: this comparison is always true (staticcheck)
  		if err := syscall.Exec(limactl, haCmd.Args, haCmd.Environ()); err != nil {
```

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
(cherry picked from commit bdf7c7f)
> Conflicts:
>	pkg/instance/start.go
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

Signed-off-by: Anders F Björklund <anders.f.bjorklund@gmail.com>
(cherry picked from commit 86b4495)
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

[release/1.2] cherry-picks