We need to make sure a device is properly added/removed (or not) to a
domain definition to check that a hotplug API did not lie to us.

The test is currently testing just device update function. However,
chardev hotplug is implemented just for device attach and detach. This
fact means, the test needs to be rewritten (the majority of the code is
still shared). Moreover, we are now able to pass VM among multiple test
runs. So for instance, while we add a device in the first run, we can
remove it in the second run.