"Restart" button crashes XQEMU

[JayFoxRox]

When hitting the "Restart" button (which sends QMP system_reset), I always seem to get:

hw/xbox/nv2a/nv2a_pfifo.c:278: pfifo_run_pusher: Assertion `false' failed.

I think we should remove the button for now, or hide/disable it (until XQEMU doesn't crash anymore).
@mborgerson has mentioned on IRC that he already fixed XQEMU system resets while working on snapshots. So if he fixes the issue in XQEMU soon, hiding the button might not be necessary.

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Additionally, the "Restart" button is always enabled, even if XQEMU is not running. It just does nothing. So if we don't remove it, we'll need a new issue about this.

[JayFoxRox]

Why do we even have a restart button in the first place? Xbox also doesn't have one. There's no meaningful way to implement it.

We should have a "Eject tray" button instead (the closest to a restart-button), but that would have side-effects and not be started using a QMP system_reset.