lava: Update QEmu audio option

Pipewire tests may fail due to audio timeouts as the emulated sound card doesn't seem to consume data.
Replace -soundhw hda option by a dummy backend that discards all outputs.
This requests QEmu version > 4.

Update session-lock-down-no-deny expected results to reflect PulseAudio replacement by PipeWire/WirePlumber.

https://phabricator.apertis.org/T8436

Signed-off-by: Frédéric Danis frederic.danis@collabora.com