use GuestMemory::read_from() and GuestMemory::write_to() in Vsock

[serban300]

As part of #2574 we refactored vsock in order to use vm-memory methods instead of raw pointers. For performance reasons and limitations related to our vm-memory wrapper crate we used GuestMemoryRegion::read_from() and GuestMemoryRegion::write_to()

After dirty bitmap tracking is merged in rust-vmm/vm-memory we should be able to use GuestMemory::read_from() and GuestMemory::write_to() directly.

[alindima]

While the performance aspect is fixed by #2661 , this issue is blocked by: rust-vmm/vm-memory#171

[roypat]

With #3653 this no longer applies