qemu: blockjob: Don't emit traditional disk events for jobs without disk

qemu: blockjob: Don't emit traditional disk events for jobs without disk

With -blockdev it will be possible that a block job loses the disk that
was used to start it to a guest-initiated hot-unplug. Don't emit the
block job events in that case as we can't report the top level source or
disk target for an unplugged (and potentially replugged with different
source) disk.

Eventually when we add machinery for tracking jobs globally for a VM the
event will be reinstated via the domain job event.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>