When formatting new qcow2 images we need to provide the backing store
string which should not contain any authentication or irrelevant data.

Add a flag for qemuBlockStorageSourceGetBackendProps which allows to
skip the irrelevant data.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>