consume i8042 device from vm-superio

[lauralt]

At this point we don't have an implementation of the i8042 device in vmm-reference, and we don't do anything for the related vcpu exits. We should consume the device from vm-superio in order to gracefully handle the shutdown.

[andreeaflorescu]

Task breakdown:

• Define an i8042Wrapper in devices/src/legacy that extends the upstream i8042. This should be defined similar to the RtcWrapper or the SerialWrapper.
• Implement the MutDevicePio trait for the i8042 Wrapper. A good example of how this can be done is here: https://github.com/google/crosvm/blob/cbddc592dd256155f14054e10b5c539c3c6c794a/devices/src/i8042.rs#L23
• in vmm/src/lib.rs initialize the i8042 wrapper and add it to the device_mgr (this should also include calling register_irqfd)

Once these things are done, we can test the integration works by removing "reboot=t" from the command line defined in vmm/src/lib.rs and check that we still properly run the tests that expect the vmm to exit.

[lauralt]

Fixed by #215