qemu-iotests: prefill some data to test image

Case 030 occasionally fails because of block job compltes too fast to be
captured by script, and 'unexpected qmp event' of job completion causes
the test failure.

Simply fill in some data to the test image to make this false alarm less
likely to happen.

(For other benefits to prefill data to test image, see also commit
ab68cdfaa).

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
