hypervisor, vmm: Add dynamic control of logging dirty pages

This patch extends slightly the current live-migration code path with
the ability to dynamically start and stop logging dirty-pages, which
relies on two new methods added to the `hypervisor::vm::Vm` Trait. This
patch also contains a complete implementation of the two new methods
based on `kvm` and placeholders for `mshv` in the `hypervisor` crate.

Fixes: #2858

Signed-off-by: Bo Chen <chen.bo@intel.com>

vmm/src/lib.rs

@@ -1049,6 +1049,9 @@ impl Vmm {
             // Send last batch of dirty pages
             Self::vm_maybe_send_dirty_pages(vm, &mut socket)?;
 
+            // Stop logging dirty pages
+            vm.stop_memory_dirty_log()?;
+
             // Capture snapshot and send it
             let vm_snapshot = vm.snapshot()?;
             let snapshot_data = serde_json::to_vec(&vm_snapshot).unwrap();