vsock: TCP test + fix a bug leading to an error in TCP connect call in guest

[mtjhrc]

This PR adds a simple tests that attempts to connect to a TCP socket on host from the guest. However this would often fail, due to a race condition:

As far as I understand, this is what happens:
libkrun connects to the TCP socket on the host (this action is done upon the TSI_CONNECT request), but the guest is not connected to stream vsock socket corresponding to the actual transport for that connection. (We have not called confirm_connect). The problem is that since we assume the guest is already connected we attempt to recv here:

libkrun/src/devices/src/virtio/vsock/tcp.rs

Lines 700 to 714 in a84dc1a

	if evset.contains(EventSet::IN) {
	debug!("process_event: IN");
	if self.status == ProxyStatus::Connected {
	let (signal_queue, wait_credit) = self.recv_pkt();
	update.signal_queue = signal_queue;
	if wait_credit && self.status != ProxyStatus::WaitingCreditUpdate {
	self.status = ProxyStatus::WaitingCreditUpdate;
	let rx = MuxerRx::CreditRequest {
	local_port: self.local_port,
	peer_port: self.peer_port,
	fwd_cnt: self.tx_cnt.0,
	};
	update.push_credit_req = Some(rx);
	}

This returns wait_credit = true, which causes us to send VSOCK_OP_CREDIT_REQUEST before sending the VSOCK_OP_RESPONSE to the VSOCK_OP_REQUEST which the guest kernel sends.
The kernel is not expecting this and returns EPROTO from connect call in the guest userspace. (specifically EPROTO is returned from the virtio_transport_recv_connecting in net/vmw_vsock/virtio_transport_common.c)

The fix here introduces a new state ConnectedUnconfirmed which it uses for TCP Proxy code in vsock. This state is used as a transitional state between the host socket being connected to libkrun and the confirm_connect being called, meaning the connection is actually established to the guest and we can actually send/recv.

A similar issue cannot happen with UDP sockets, because VSOCK_OP_REQUEST/VSOCK_OP_RESPONSE is only used for stream sockets. This also cannot currently happen with UNIX stream sockets, because currently these are only ever connected on the host and confirm_connect-ed at the same time here:

libkrun/src/devices/src/virtio/vsock/muxer.rs

Lines 533 to 534 in a84dc1a

	let update = unix.connect(pkt, tsi);
	unix.confirm_connect(pkt);

Though, should the unix stream sockets also use the ConnectedUnconfirmed state too? For consistency of the semantics of the Proxy trait methods it would make sense...

Depends on: #258