qemu-iotests: Adjustments due to error propagation

When opening/creating images, propagating errors instead of immediately
emitting them on occurrence results in errors generally being printed on
a single line rather than being split up into multiple ones. This in
turn requires adjustments to some test results.

Also, test 060 used a sed to filter out the test image directory and
format by removing everything from the affected line after a certain
keyword; this now also removes the error message itself, which can be
fixed by using _filter_testdir and _filter_imgfmt.

Finally, _make_test_img in common.rc did not filter out the test image
directory etc. from stderr. This has been fixed through a redirection of
stderr to stdout (which is already done in _check_test_img and
_img_info).

Signed-off-by: Max Reitz <mreitz@redhat.com>