[virtio-balloon] Add support for Free Page Hinting and Reporting

[JackThomson2]

This issue to track the development adding free page hinting and reporting to the virtio-balloon device in firecracker.
Free page hinting and reporting are a mechanism to notify the host of unused memory in the guest, allowing the host to free this memory potentially reducing the RSS of a VM, to increase density or reduce snapshot size.

Firecracker support for these features will include:

• Ability to enable free page reporting
• Ability to start, monitor and stop free page hinting runs