qemu-img: Report error even with --oformat=json

img_check() should report that the format of the given image does not
support checks even if JSON output is desired. JSON data is output to
stdout, as opposed to error messages, which are (in the case of
qemu-img) printed to stderr. Therefore, it is easy to distinguish
between the two.

Also, img_info() does already use error_report() for human-readable
messages even though JSON output is desired (through
collect_image_info_list()).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>