Fix soundness issues with MMIO and shared memory

[qwandor]

The way the volatile crate handles MMIO regions is not sound, because it creates references to memory which shouldn't be considered dereferenceable. There is an attempt to fix this in rust-osdev/volatile#22 (and lots of discussion), but that PR has been open since May 2021 and doesn't seem near being submitted. Instead, I've implemented the functionality we need in a new volatile module here using the addr_of! and addr_of_mut! macros, keeping all MMIO regions as pointers and never creating references.

This works for the MMIO transport and config regions. For the Virtqueue, I don't think we need volatile access at all because it's just shared memory, not MMIO. We still shouldn't be using references, however, as DMA to these shared regions by the device violates Rust's aliasing rules, so I've changed the implementation to use pointers instead.

[wangrunji0408]

LGTM. I didn't realize this problem before. Thanks for your work!