The storage volume may in fact convert into a directory when starting
the VM so that it may be actually possible to use it.

This is a regression caused by c9b27af3 as moving the check to
validation time without adjustment causes problems as the volumes are
not translated yet.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>