vm-migration, vmm: Send configuration in separate step

Prior to sending the memory the full state is not needed only the
configuration. This is sufficient to create the appropriate structures
in the guest and have the memory allocations ready for filling.

Update the protocol documentation to add a separate config step and move
the state to after the memory is transferred. As the VM is created in a
separate step to restoring it the requires a slightly different
constructor as well as saving the VM object for the subsequent commands.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vmm/src/vm.rs

@@ -732,6 +732,40 @@ impl Vm {
         )
     }
 
+    pub fn new_from_migration(
+        config: Arc<Mutex<VmConfig>>,
+        exit_evt: EventFd,
+        reset_evt: EventFd,
+        seccomp_action: &SeccompAction,
+        hypervisor: Arc<dyn hypervisor::Hypervisor>,
+    ) -> Result<Self> {
+        #[cfg(target_arch = "x86_64")]
+        hypervisor.check_required_extensions().unwrap();
+        let vm = hypervisor.create_vm().unwrap();
+        #[cfg(target_arch = "x86_64")]
+        vm.enable_split_irq().unwrap();
+        let phys_bits = physical_bits(config.lock().unwrap().cpus.max_phys_bits);
+
+        let memory_manager = MemoryManager::new(
+            vm.clone(),
+            &config.lock().unwrap().memory.clone(),
+            false,
+            phys_bits,
+        )
+        .map_err(Error::MemoryManager)?;
+
+        Vm::new_from_memory_manager(
+            config,
+            memory_manager,
+            vm,
+            exit_evt,
+            reset_evt,
+            seccomp_action,
+            hypervisor,
+            None,
+        )
+    }
+
     fn load_initramfs(&mut self, guest_mem: &GuestMemoryMmap) -> Result<arch::InitramfsConfig> {
         let mut initramfs = self.initramfs.as_ref().unwrap();
         let size: usize = initramfs