migration/postcopy: Ensure postcopy_start() sets errp if it fails

There are several places where postcopy_start() fails without setting
errp. This can cause a null pointer de-reference, as in case of error,
the caller of postcopy_start() copies/prints the error set in errp.

Fix it by setting errp in all of postcopy_start() error paths.

Cc: qemu-stable <qemu-stable@nongnu.org>
Fixes: 908927d ("migration: Update error description whenever migration fails")
Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20240328140252.16756-3-avihaih@nvidia.com
Signed-off-by: Peter Xu <peterx@redhat.com>

migration/migration.c

@@ -2510,6 +2510,8 @@ static int postcopy_start(MigrationState *ms, Error **errp)
         migration_wait_main_channel(ms);
         if (postcopy_preempt_establish_channel(ms)) {
             migrate_set_state(&ms->state, ms->state, MIGRATION_STATUS_FAILED);
+            error_setg(errp, "%s: Failed to establish preempt channel",
+                       __func__);
             return -1;
         }
     }
@@ -2525,17 +2527,22 @@ static int postcopy_start(MigrationState *ms, Error **errp)
 
     ret = migration_stop_vm(ms, RUN_STATE_FINISH_MIGRATE);
     if (ret < 0) {
+        error_setg_errno(errp, -ret, "%s: Failed to stop the VM", __func__);
         goto fail;
     }
 
     ret = migration_maybe_pause(ms, &cur_state,
                                 MIGRATION_STATUS_POSTCOPY_ACTIVE);
     if (ret < 0) {
+        error_setg_errno(errp, -ret, "%s: Failed in migration_maybe_pause()",
+                         __func__);
         goto fail;
     }
 
     ret = bdrv_inactivate_all();
     if (ret < 0) {
+        error_setg_errno(errp, -ret, "%s: Failed in bdrv_inactivate_all()",
+                         __func__);
         goto fail;
     }
     restart_block = true;
@@ -2612,6 +2619,7 @@ static int postcopy_start(MigrationState *ms, Error **errp)
 
     /* Now send that blob */
     if (qemu_savevm_send_packaged(ms->to_dst_file, bioc->data, bioc->usage)) {
+        error_setg(errp, "%s: Failed to send packaged data", __func__);
         goto fail_closefb;
     }
     qemu_fclose(fb);