Allowing a NULL @secretPath for virStorageBackendCreateQemuImgCmdFromVol
would result in a generated command line with a dangling "file=" output.
So let's make sure the @secretPath exists before processing.

This means we should pass a dummy path from the storage test.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>
ACKed-by: NMichal Privoznik <mprivozn@redhat.com>