vm-device: Add new crate for virtio and VFIO agnostic traits

The new crate vm-device is created here to host the definitions of
traits not meant to be tied to virtio of VFIO specifically. We need to
add a new trait to update external DMA mappings for devices, which is
why the vm-device crate is the right fit for this.

We can expect this crate to be extended later once the design gets
approved from a rust-vmm perspective.

In this specific use case, we can have some devices like VFIO or
vhost-user ones requiring to be notified about mapping updates. This
new trait ExternalDmaMapping will allow such devices to implement their
own way to handle such event.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

Cargo.lock

@@ -183,6 +183,7 @@ dependencies = [
  "vhost_rs 0.1.0",
  "vhost_user_backend 0.1.0",
  "virtio-bindings 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "vm-device 0.1.0",
  "vm-memory 0.1.0 (git+https://github.com/rust-vmm/vm-memory)",
  "vm-virtio 0.1.0",
  "vmm 0.1.0",
@@ -1022,6 +1023,10 @@ dependencies = [
  "vm-memory 0.1.0 (git+https://github.com/rust-vmm/vm-memory)",
 ]
 
+[[package]]
+name = "vm-device"
+version = "0.1.0"
+
 [[package]]
 name = "vm-memory"
 version = "0.1.0"