migration: Unwrap qemu_savevm_state_complete_precopy() in postcopy

Postcopy invokes qemu_savevm_state_complete_precopy() twice for a long
time, and that caused way too much confusions.  Let's clean this up and
make postcopy easier to read.

It's actually fairly straightforward: postcopy starts with saving
non-postcopiable iterables, then later it saves again with non-iterable
only.  Move these two calls out makes everything much easier to follow.
Otherwise it's very unclear what qemu_savevm_state_complete_precopy() did
in either of the calls.

No functional change intended.

Signed-off-by: Peter Xu <peterx@redhat.com>
Tested-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
Link: https://lore.kernel.org/r/20250114230746.3268797-13-peterx@redhat.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>

migration/migration.c

@@ -2693,7 +2693,11 @@ static int postcopy_start(MigrationState *ms, Error **errp)
      * Cause any non-postcopiable, but iterative devices to
      * send out their final data.
      */
-    qemu_savevm_state_complete_precopy(ms->to_dst_file, true);
+    ret = qemu_savevm_state_complete_precopy_iterable(ms->to_dst_file, true);
+    if (ret) {
+        error_setg(errp, "Postcopy save non-postcopiable iterables failed");
+        goto fail;
+    }
 
     /*
      * in Finish migrate and with the io-lock held everything should
@@ -2732,7 +2736,12 @@ static int postcopy_start(MigrationState *ms, Error **errp)
      */
     qemu_savevm_send_postcopy_listen(fb);
 
-    qemu_savevm_state_complete_precopy(fb, false);
+    ret = qemu_savevm_state_complete_precopy_non_iterable(fb, true);
+    if (ret) {
+        error_setg(errp, "Postcopy save non-iterable device states failed");
+        goto fail_closefb;
+    }
+
     if (migrate_postcopy_ram()) {
         qemu_savevm_send_ping(fb, 3);
     }

migration/savevm.c

@@ -1477,7 +1477,6 @@ void qemu_savevm_state_complete_postcopy(QEMUFile *f)
     qemu_fflush(f);
 }
 
-static
 int qemu_savevm_state_complete_precopy_iterable(QEMUFile *f, bool in_postcopy)
 {
     int64_t start_ts_each, end_ts_each;

migration/savevm.h

@@ -44,6 +44,7 @@ void qemu_savevm_state_pending_exact(uint64_t *must_precopy,
                                      uint64_t *can_postcopy);
 void qemu_savevm_state_pending_estimate(uint64_t *must_precopy,
                                         uint64_t *can_postcopy);
+int qemu_savevm_state_complete_precopy_iterable(QEMUFile *f, bool in_postcopy);
 void qemu_savevm_send_ping(QEMUFile *f, uint32_t value);
 void qemu_savevm_send_open_return_path(QEMUFile *f);
 int qemu_savevm_send_packaged(QEMUFile *f, const uint8_t *buf, size_t len);