qemu-iotests: fill streaming test image with data

qemu-iotests: fill streaming test image with data

The TestStreamStop test case is racy; if the job completes before we can
cancel it, it fails.  If we remove the sleep the job will be canceled
before it has even started, and the test succeeds but it is also not
testing anything interesting.

But if the image is left sparse, then the job has really nothing to do.
For qcow2 it will read one L2-table, for raw it will issue a bunch of
ioctls.  This also falls under "not testing anything interesting", and
this may be happening right now (depending on the filesystem) since the
file protocol got an is_allocated method.

Filling the test image with data ensures that the test covers the
intended case.  It also slows down the test, which will be particularly
important after the next patch.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>