qdev: make release_string() idempotent

So it can eventually be called multiple times safely.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Link: https://lore.kernel.org/r/20250429140306.190384-2-marcandre.lureau@redhat.com
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20260108230311.584141-3-csomani@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
1 file changed