Replace memory-model with vm-memory from rust-vmm

[andreeaflorescu]

This task is currently blocked by vm-memory not being published on crates.io.
Check the repository here: https://github.com/rust-vmm/vm-memory

@bonzini did the switch in his fork, but it's not ready for merging in Firecracker as per his comment.

[serban300]

This seems a duplicate of #1137 . I think we can close it.