The vm_memory implementation for VolatileSlice is able to read and write
to a source or destination which implements a Read or Write trait.
Unfortunately, this is not enough for this specific use case as we need
to be able to write to a file at a specific offset, which is not
provided by the Read or Write trait.
This code has been ported over from crosvm commit
961461350c0b6824e5f20655031bf6c6bf6b7c30.
Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
