nixos/tests/vbox: Make GUI startup more robust.

Sometimes, keys aren't properly recognized the first time, so in order
to make sure they get through, always resend the key again on retry.

In this case the worst that could happen is that the VM is started over
and over again, but never in parallel, so that's fine because we're
checking for successful startup 10 seconds after the keypress.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>