This commit refactors DiskFormat to turn it into a proper
factory class and to also include type hints to facilitate it's
use from an API POV. Related to #1498
The creation of the qcow2 format was done in one qemu-img
convert call. That call instructs qemu to compress and
convert in one call. The downside of this approach is that
not all qcow2 options can be used. For example the setup
of:
<type ... formatoptions="preallocation=metadata"/>
failed the build with an error message that compression and
preallocation is not possible at the same time. Thus this patch
changes the way the qcow2 image is created to be done in two
steps. The first step converts the format without compression
and therefore allows for any format option to be used. The
second call only applies the compression and leads to the
final result.
