template.py, image.py: create volumes and backing-store with proper format

There was confusion between the 'front' file and the 'backing-store'
file formats, causing front files to be created with type 'raw'
when they really must be qcow2.

1) volume() doesn't need to create raw; it's only used for front files
and they must be qcow2.  (removing the code removes a typo,
_format vs. format_, too).
2) sparse can never be True in volume_clone() as written; remove code
3) compress the 'raw' test for the backing-store type in volume_clone

Signed-off-by: Dan Mick <dan.mick@inktank.com>