kvm tools: Add BUG_ON() helper to make a run-time critical tests

Also drop useless assert.h inclusions.

Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>