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. You will need 3 terminals:

1. (Only needs to be run once): Prepare rootfs and guest kernel: get_rootfs_guest_kernel.sh
2. [Terminal A] Run firecracker: run_firecracker.sh
3. [Terminal B] Initialize a VM with a balloon and snapshot it: snapshot_vm.sh
4. [Terminal A] You will probably need to kill the former firecracker process and restart it: run_firecracker.sh
5. [Terminal C] Start the UFFD handler with the snapshot: run_uffd_handler.sh

• Sometimes there are permissions issues if you previously used sudo tools/devtool build to build a custom firecracker version because the UFFD handler is not run as root. I sudo rm -rf build/cargo_target/debug and use the script to rebuild the artifacts as the current user

7. [Terminal B] 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