runtime.bash: fix check for parameter files

We need to check if the file exists, not just if it is a non-empty string.
While an empty $path would have the unfortunate effect that "cat" would
read from stdin, that is not an issue as you can simply not do that.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>