vm-migration: Define the Snapshottable and Transportable traits

A Snapshottable component can snapshot itself and
provide a MigrationSnapshot payload as a result.

A MigrationSnapshot payload is a map of component IDs to a list of
migration sections (MigrationSection). As component can be made of
several Migratable sub-components (e.g. the DeviceManager and its
device objects), a migration snapshot can be made of multiple snapshot
itself.
A snapshot is a list of migration sections, each section being a
component state snapshot. Having multiple sections allows for easier and
backward compatible migration payload extensions.

Once created, a migratable component snapshot may be transported and this
is what the Transportable trait defines, through 2 methods: send and recv.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Yi Sun <yi.y.sun@linux.intel.com>

vm-virtio/src/transport/mmio.rs

@@ -15,8 +15,8 @@ use std::result;
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::{Arc, Mutex};
 use vm_device::interrupt::InterruptSourceGroup;
-use vm_device::{Migratable, MigratableError, Pausable, Snapshotable};
 use vm_memory::{GuestAddress, GuestAddressSpace, GuestMemoryAtomic, GuestMemoryMmap};
+use vm_migration::{Migratable, MigratableError, Pausable, Snapshottable, Transportable};
 use vmm_sys_util::{errno::Result, eventfd::EventFd};
 
 const VENDOR_ID: u32 = 0;
@@ -354,5 +354,6 @@ impl Pausable for MmioDevice {
     }
 }
 
-impl Snapshotable for MmioDevice {}
+impl Snapshottable for MmioDevice {}
+impl Transportable for MmioDevice {}
 impl Migratable for MmioDevice {}