virtio_queue: fix license in Cargo.toml

[MatiasVara]

Summary of the PR

Make Cargo.toml reflect the Apache-2.0 AND BSD-3-Clause license. This PR is regarding to #283.

Requirements

Before submitting your PR, please make sure you addressed the following requirements:

• [x] All commits in this PR have Signed-Off-By trailers (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
• [x] All added/changed functionality has a corresponding unit/integration test.
• [x] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• [x] Any newly added unsafe code is properly documented.

[stefano-garzarella]

@MatiasVara thanks for this!

I'd suggest "virtio_queue: fix license in Cargo.toml" as title, to make it clearer that we are fixing it and not changing it. Because if we change it we should have an agreement with everyone who contributed.

I also suggest explaining better in the commit description what was the problem (e.g. that almost all files have SPDX with the AND but the Cargo.toml did not reflect this.)

Please also add Closes #283 before SOB, in this way when we merge the issue will be closed automatically.