Since commit 7140807917, qemu agent
channel cannot be plugged in because we won't generate its path
automatically. Let's not only fix that, but also add tests for it so
next time it's checked for.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1322210
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
