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.

Each test case label now contains more data useful to identify the test.

If testQemuHotplugAttach succeeds, the vm->def steals the dev pointer.
However, not the envelope, which needs to be freed. In addition,
driver.config is allocated, but never freed.

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.

As my punishment for the break in 7f15ebc7 (fixed in 752596b5) I'm
introducing this test to make sure it won't happen again. Currently,
only test for <graphics/> is supported.