lava: Fix typo in LAVA 2021.05 workarounds

Commit "lava: Work around LAVA 2021.05 regression with kernel_start_message"
1bb8ba90 still spelled `kernel_start_message` in the QEMU boot
template while the correct spelling is now `kernel-start-message`, as
used by other templates.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>