support creating raw disk image

vmbuilder spits out qcow2 images by default. Generally, that's fine and dandy. However, it would be nice to have it create raw images instead.

As a quick hack, I was able to do this by changing:

--- vm-builder-0.12.3.orig/VMBuilder/plugins/kvm/vm.py
+++ vm-builder-0.12.3/VMBuilder/plugins/kvm/vm.py
@@ -26,7 +26,7 @@
 class KVM(Hypervisor):
     name = 'KVM'
     arg = 'kvm'
- filetype = 'qcow2'
+ filetype = 'raw'
     preferred_storage = VMBuilder.hypervisor.STORAGE_DISK_IMAGE
     needs_bootloader = True

Clearly this isn't correct, but it would be nice if that filetype parameter was an option, and allow the user to input 'raw' the desired file type.

Alternatively, if just qcow2 and raw are the only supported types, it would just be a matter of skipping the:
            img_path = disk.convert(destdir, self.filetype)
step in that same file.

Thanks.