`DriverClient.Start()` should wait execution of actual driver's `Start()`

[norio-nomura]

#4343 (comment)

I checked the operation with the debugger. The real problem is that DriverClient.Start() does not wait for the execution of LimaVzDriver.Start(). As a result, AdditionalSetupForSSH() is called and hangs up before the waiting channel is set up in Start(). DriverClient.Start() and DriverServer.Start() should be modified.