iotests: Make 191 work with qcow2 options

In order for 191 to work with an explicit refcount_bits or compat=0.10,
we should strip format-specific information from the output--and we can
do so by using _filter_img_info.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20171123020832.8165-17-mreitz@redhat.com
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>