[KVM] Add support for alternate disk formats

Allows the user to define a Libvirt-supported disk_format
(ex. qcow2, raw, vmdk) in the definition.rb. Existing
templates that use the VDI disk_format will instead be
treated as 'raw'.
Merge pull request #507 from DavidWittman/kvm-disk-format

[KVM] Add support for alternate disk formats