qemu: domain: Use 'raw' for 'volume' disks without format

qemu: domain: Use 'raw' for 'volume' disks without format

Storage pools might want to specify format of the image when translating
the volume thus we can't add any default format when parsing the XML.

Add a explicit format when starting the VM and format is not present
neither by user specifying it nor by the storage pool translation
function.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>