util: storagefile: Don't traverse storage sources unusable by VM

util: storagefile: Don't traverse storage sources unusable by VM

virStorageFileGetMetadataRecurse would include files in the backing
chain which would not really be usable by libvirt directly e.g.
when such file would be promoted to the top layer by an active block
commit as for example inline authentication data can't be represented in
the VM xml file. The idea is to use secrets for this.

With the changes to the backing store string parsers we can report and
propagate if such a thing is present in the configuration and thus start
skipping those files in the backing chain traversal code. This approach
still allows to report the appropriate backing store string in the
storage driver which doesn't directly use the backing file.

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