vm-device: Initial Migratable trait

The Migratable trait groups all expected capabilities of devices and
components that can be migrated.

For a component to be migrated, it must be able to pause and resume.
Once paused, it should be able to provide a snapshot of itself. It
should also be able to restore itself from a snaphot.

As a consequence, the Migratable trait will be split between the
Pausable and the Snapshotable traits. This commit only adds the
Pausable one.

All migratable devices will be tracked from the DeviceManager.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

vm-device/Cargo.toml

@@ -5,3 +5,8 @@ authors = ["The Cloud Hypervisor Authors"]
 edition = "2018"
 
 [dependencies]
+anyhow = "1.0"
+thiserror = "1.0"
+serde = {version = ">=1.0.27", features = ["rc"] }
+serde_derive = ">=1.0.27"
+serde_json = ">=1.0.9"