block: add abstraction for file access

[lauralt]

This PR proposes a simple (but quite ugly) abstraction for the virtio block request execution (and is also tied to the std::io::Read and std::io::Write traits).

We should add an abstraction for nicely accessing the file sectors, an example can be found in this comment. This way, more efficient/complex file access APIs, such as pread/pwrite or io_uring could be enabled as well.