[RFC] vfio-bindings: Add new bindings from kernel v6.6 (for supporting VFIO cdev uAPIs)

[likebreath]

The vfio-bindings crate was generated from kernel v5.0 several years ago. To support vfio cdev (#92), we will need to use at least kernel v6.6. Here is the proposal to add such support:

1. Generate new bindings base on kernel v6.6;
2. Put the new bindings in a new folder vfio-bindings/src/bindings_v6_6_0;
3. Put the new bindings behind a feature flag bindings_v6_6_0;
4. Make bindings_v5_0_0 default explicitly to avoid unexpected breakage from the crate consumers (say Cloud Hypervisor) Use compile-time checks to ensure one and only one version of vfio bindings is enabled.

Any questions or comments, please let me know. Thanks.