Introduce helpers to split a virtio descriptor into its readable part on
one side, and into its writable part on the other side. This is useful
to separate the request from the reply.
This code has been ported over from crosvm commit
961461350c0b6824e5f20655031bf6c6bf6b7c30.
Two important modifications have been applied to the original code:
- GuestMemory is replaced by GuestMemoryMmap from the vm-memory crate,
which comes with different ways of accessing the memory regions.
- VolatileSlice has different methods, which means the code has been
updated accordingly.
Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
