Provide atomic accesses to guest memory

[alexandruag]

We should fix logic that handles guest memory access and does not guarantee the operations will have volatile semantics. One example is the copy here:

vm-memory/src/volatile_memory.rs

Line 277 in 08b24bf

copy(self.addr, slice.addr, min(self.size, slice.size));

[bonzini]

Hmm, this is not actually what I wanted to work on. The issue is that the rust compiler (and the processor on weakly-ordered architectures) need to be told about possibly concurrent access, and they should be properly annotated as "atomic" rather than "volatile". My idea here is to add a VolatileAtomicRef struct similar to VolatileRef, but supporting atomic load and store instead of Deref and DerefMut.

[alexandruag]

Sorry about that :( For some reason I thought this one implies the other as well, but even if it did, the description was not clear at all in this regard. Assigned myself for this one.