iotests: Drop format-specific in _filter_img_info

iotests: Drop format-specific in _filter_img_info

_filter_img_info should remove format-specific information, too.  We
already have such a filter in _img_info, and it is very useful for
query-block-named-block-nodes (etc.), too.

However, in 198 we need that information (but we still want the rest of
the filter), so make that filtering optional.  Note that "the rest of
the filter" includes filtering of the test directory, so we can drop the
_filter_testdir from 198 at the same time.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20171123020832.8165-7-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>