Update copyright headers for @josephlr contributions

[rbradford]

Google's preferred policy on this is actually public: https://opensource.google/documentation/reference/patching#license_headers_and_copyright_notices

When contributing to third-party projects, Googlers do not need to add Google's copyright notice when authoring patches to existing files. Googlers should add Google's copyright notice (or a "The Project Authors" style copyright notice) to new files being added to the library if permitted by the project maintainers.

So either:

// SPDX-License-Identifier: Apache-2.0
// Copyright <year> Google LLC.

or

// SPDX-License-Identifier: Apache-2.0
// Copyright <year> The Rust Hypervisor Firmware Authors

would work for any files I created.

Originally posted by @josephlr in #256 (comment)

[rbradford]

I've reviewed the changes in #256 / merged to main and to me it looks like it matches this (first version.)