iotests: Use _img_info

qemu-img info should only be used directly if the format-specific
information or the name of the format is relevant (some tests explicitly
test format-specific information; test 082 uses qcow2-specific settings
to test the qemu-img interface); otherwise, tests should always use
_img_info instead.

Test 082 was touched only partially. It does test the qemu-img
interface; however, its invocations of qemu-img info are not real tests
but rather verifications, so if format-specific information is not
important for the test, there is no reason not to use _img_info. In
contrast to directly invoking qemu-img info, "qcow2" is replaced by
"IMGFMT"; but as "qcow2" is only mentioned once in test 082 (in
_supported_fmt), I consider this an improvement.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1412105489-7681-2-git-send-email-mreitz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
