kvm: Better handle different volume formats when creating a volume

The KVMStorageProcessor no longer has a hardcoded if-statement which sets
RBD volumes to RAW, this is now handled in the LibvirtStorageAdapter

The Management Server still sends QCOW2 as format. That's a fix for later.
CLOUDSTACK-4549: Do not force RBD snapshot backups to RAW format

Since we use qemu-img to copy from RBD to Secondary Storage we no
longer have to force to RAW images, but can stick with QCOW2

When the snapshot backups are QCOW2 format they can easily be deployed
again when restoring from a backup