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: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>