Introduce virStorageSourceSlice which will store the 'offset' and 'size'
of a virStorageSource and declare it as 'sliceStorage' and 'sliceFormat'
attributes of a virStorageSource.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>