Bump version to 0.7.4
Rename **kargs to **kwargs

This complies better with the python conventions
Miror fixes in comments
Use the device mapper to snapshot raw images

Do not use qcow2 to snapshot images that are raw. qemu-nbd is not
present in some old systems like CentOS 6.x. All we can do to support
those systems is to use the device mapper in as many cases as possible.
Handle the case where qemu-nbd is missing

If qemu-nbd is not present at the host system, only accept raw images
as input media. This is a workaround for ancient systems like CentOS 6.x
where the nbd module is not compiled in the default kernel and the qemu
package does not include the qemu-nbd executable.
Update ChangeLog and version.py for 0.7.4
Merge branch 'hotfix-0.7.4'