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>

vm-device/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "vm-device"
+version = "0.1.0"
+authors = ["The Cloud Hypervisor Authors"]
+edition = "2018"
+
+[dependencies]