qemu-iotests: extract wait_until_completed() into iotests.py

qemu-iotests: extract wait_until_completed() into iotests.py

The 'drive-mirror' tests often issue 'block-job-complete' and wait for
the QMP completion event.  Other types of block jobs also want to wait
for completion but they may not need to issue 'block-job-complete'.

Extract wait_until_completed() from 041 and put it into iotests.py.
Return the QMP event object so the caller can make additional
assertions, if necessary.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>