migration: Add MIG_CMD_SWITCHOVER_START and its load handler

This QEMU_VM_COMMAND sub-command and its switchover_start SaveVMHandler is
used to mark the switchover point in main migration stream.

It can be used to inform the destination that all pre-switchover main
migration stream data has been sent/received so it can start to process
post-switchover data that it might have received via other migration
channels like the multifd ones.

Add also the relevant MigrationState bit stream compatibility property and
its hw_compat entry.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Zhang Chen <zhangckid@gmail.com> # for the COLO part
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Link: https://lore.kernel.org/qemu-devel/311be6da85fc7e49a7598684d80aa631778dcbce.1741124640.git.maciej.szmigiero@oracle.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

include/migration/client-options.h

@@ -10,6 +10,10 @@
 #ifndef QEMU_MIGRATION_CLIENT_OPTIONS_H
 #define QEMU_MIGRATION_CLIENT_OPTIONS_H
 
+
+/* properties */
+bool migrate_send_switchover_start(void);
+
 /* capabilities */
 
 bool migrate_background_snapshot(void);

include/migration/register.h

@@ -279,6 +279,18 @@ typedef struct SaveVMHandlers {
      * otherwise
      */
     bool (*switchover_ack_needed)(void *opaque);
+
+    /**
+     * @switchover_start
+     *
+     * Notifies that the switchover has started. Called only on
+     * the destination.
+     *
+     * @opaque: data pointer passed to register_savevm_live()
+     *
+     * Returns zero to indicate success and negative for error
+     */
+    int (*switchover_start)(void *opaque);
 } SaveVMHandlers;
 
 /**