xen-bindings: add SAFETY docs to unsafe{}s blocks

The default CI pipeline for rust-vmm unconditionally errors for
undocumented unsafe blocks, so workaround the problem for now by adding
them manually in bindgen-generated methods.

The proper solution would be to use a custom pipeline for this
repository.

Signed-off-by: Manos Pitsidianakis <[email protected]>

Author: epilys