vm-virtio: Trigger external map/unmap from virtio-iommu

This patch relies on the trait implementation provided for each device
which requires some sort of external update based on a map or unmap.

Whenever a MAP or UNMAP request comes through the virtqueues, it
triggers a call to the external mapping trait with map()/unmap()
functions being invoked.

Those external mappings are meant to be used from VFIO and vhost-user
devices as they need to update their own mappings. In case of VFIO, the
goal is to update the DMAR table in the physical IOMMU, while vhost-user
devices needs to update their internal representation of the virtqueues.

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

vm-virtio/Cargo.toml

@@ -21,6 +21,7 @@ pci = { path = "../pci", optional = true }
 tempfile = "3.1.0"
 virtio-bindings = { git = "https://github.com/rust-vmm/virtio-bindings", version = "0.1", features = ["virtio-v5_0_0"]}
 vm-allocator = { path = "../vm-allocator" }
+vm-device = { path = "../vm-device" }
 vmm-sys-util = "0.1.1"
 
 [dependencies.vhost_rs]