iotests: Filter compat-dependent info in 198

There is a bit of image-specific information which depends on the qcow2
compat level.  Filter it so that 198 works with compat=0.10 (and any
refcount_bits value).

Note that we cannot simply drop the --format-specific switch because we
do need the "encrypt" information.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20171123020832.8165-18-mreitz@redhat.com
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>