tests: Introduce generic device hot-plug/hot-unplug functions

A lot of tests provide code for adding and removing a device via the
device_add and device_del QMP commands. Maintaining this code in so many
places is cumbersome and error-prone (some of the code parts check the
responses for device deletion in an incorrect way, for example, we've got
to deal with both, error code and DEVICE_DEL event here). So let's provide
some proper generic functions for adding and removing a device instead.

The code for correctly unplugging a device has been taken from a patch
from Peter Xu.

Reviewed-by: Peter Xu <peterx@redhat.com>
Tested-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
