iotests: Allow supported and unsupported formats at the same time

This is useful for specifying 'generic' as supported (which includes
only writable image formats), but still excluding some incompatible
writable formats.

It also removes more lines than it adds.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200924152717.287415-31-kwolf@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>