'virStorageFileChainLookup' reports an error when the lookup of the
backing chain entry is unsuccessful. Since we possibly use it multiple
times when looking up backing for 'disk->mirror' the function can report
error which won't be actually reported.
Replace the call to virStorageFileChainLookup by lookup in the chain by
index.
Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Ján Tomko <jtomko@redhat.com>