kata 2.0: delete use_vsock option and proxy abstraction

With kata containers moving to 2.0, (hybrid-)vsock will be the only
way to directly communicate between host and agent.
And kata-proxy as additional component to handle the multiplexing on
serial port is also no longer needed.
Cleaning up related unit tests, and also add another mock socket type
`MockHybridVSock` to deal with ttrpc-based hybrid-vsock mock server.

Fixes: #389

Signed-off-by: Penny Zheng penny.zheng@arm.com

src/runtime/virtcontainers/qemu_test.go

@@ -57,7 +57,7 @@ func testQemuKernelParameters(t *testing.T, kernelParams []Param, expected strin
 }
 
 func TestQemuKernelParameters(t *testing.T) {
-	expectedOut := fmt.Sprintf("panic=1 nr_cpus=%d agent.use_vsock=false foo=foo bar=bar", MaxQemuVCPUs())
+	expectedOut := fmt.Sprintf("panic=1 nr_cpus=%d foo=foo bar=bar", MaxQemuVCPUs())
 	params := []Param{
 		{
 			Key:   "foo",