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.toml

@@ -15,6 +15,7 @@ net_util = { path = "net_util" }
 vhost_user_backend = { path = "vhost_user_backend"}
 virtio-bindings = "0.1.0"
 vmm = { path = "vmm" }
+vm-device = { path = "vm-device" }
 vm-memory = { git = "https://github.com/rust-vmm/vm-memory" }
 vmm-sys-util = "0.1.1"
 vm-virtio = { path = "vm-virtio" }