qcow2: Change default for new images to compat=1.1

qcow2: Change default for new images to compat=1.1

By the time that qemu 1.7 will be released, enough time will have passed
since qemu 1.1, which is the first version to understand version 3
images, that changing the default shouldn't hurt many people any more
and the benefits of using the new format outweigh the pain.

qemu-iotests already runs with compat=1.1 by default.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>