storageDriverAutostartCallback: Refactor control flow
Use early returns to decrease the indentation level and make it more
obvious that the 'cleanup' path is a noop in those cases.
'virStoragePoolObjSetStarting' was called only when the code wanted to
start the pool, so if that was skipped, cleanup is noop as it's
conditional on the return value of 'virStoragePoolObjIsStarting'.
Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Ján Tomko <jtomko@redhat.com>