misc: Clean up license notices and copyrights

[retrage]

This PR proposes:

• Replace Apache license texts with SPDX license identifiers.
• Add missing license and copyright notices.

Some of the code files originally written by @josephlr do not have copyright notices. I'd like to confirm this with him, as I'm not sure if these contributions were made as Google or individually.

[rbradford]

Thanks @retrage!

[rbradford]

I think it's probably more appropriate to assume Google copyright here as it was submitted under that email address.

[retrage]

Thanks for your comment. Changed with commit 32c46d8.

[josephlr]

I'm sorry I missed this PR (I've moved to a new team at work and have been swamped). The Copyright is Google LLC.

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.