vmspawn: don't abort VM launch when an optional swtpm fails to start

vmspawn: don't abort VM launch when an optional swtpm fails to start

When start_tpm() failed under TPM autodetect (arg_tpm <= 0), the swtpm
block logged "Failed to start tpm, ignoring" but then fell through to
event_add_child_pidref() with the still-unset child PidRef. That returns
-ESRCH (PIDREF_NULL fails its pidref_is_set() guard), and the VM launch
fails. Register the child only on start_tpm() success.

Co-developed-by: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Paul Meyer <katexochen0@gmail.com>