qemu-iotests: Enable write tests for parallels

Originally we added parallels as a read-only format to qemu-iotests
where we did just some tests with a binary image. Since then, write and
image creation support has been added to the driver, so we can now
enable it in _supported_fmt generic.

The driver doesn't support migration yet, though, so we need to add it
to the list of exceptions in 181.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>