Add test scripts to test UFFD handler + balloon interactions

[maggie-lou]

This PR is not intended to be merged - it is just a demonstration of this issue: #4988

This PR adds logging to the UFFD handler example when the it receives a remove event.

Steps to recreate the problem:

1. Run firecracker: run_firecracker.sh
2. Initialize a VM with a balloon and snapshot it: snapshot_vm.sh
3. Start the UFFD handler with the snapshot: run_uffd_handler.sh
4. Expand the balloon. We'd expect the UFFD handler to receive EVENT_REMOVE events: trigger_remove_events.sh

On firecracker v1.10.0, the UFFD handler does not receive any EVENT_REMOVE events. With this patch, it will receive REMOVE events