migration: push Error **errp into loadvm_postcopy_handle_run()

This is an incremental step in converting vmstate loading
code to report error via Error objects instead of directly
printing it to console/monitor.
It is ensured that loadvm_postcopy_handle_run() must report an error
in errp, in case of failure.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Arun Menon <armenon@redhat.com>
Tested-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Link: https://lore.kernel.org/r/20250918-propagate_tpm_error-v14-17-36f11a6fb9d3@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

migration/savevm.c

diff --git a/migration/savevm.c b/migration/savevm.c
index ce88f56..f794716 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -2273,13 +2273,13 @@
 }
 
 /* After all discards we can start running and asking for pages */
-static int loadvm_postcopy_handle_run(MigrationIncomingState *mis)
+static int loadvm_postcopy_handle_run(MigrationIncomingState *mis, Error **errp)
 {
     PostcopyState ps = postcopy_state_get();
 
     trace_loadvm_postcopy_handle_run();
     if (ps != POSTCOPY_INCOMING_LISTENING) {
-        error_report("CMD_POSTCOPY_RUN in wrong postcopy state (%d)", ps);
+        error_setg(errp, "CMD_POSTCOPY_RUN in wrong postcopy state (%d)", ps);
         return -1;
     }
 
@@ -2626,11 +2626,7 @@
         return loadvm_postcopy_handle_listen(mis, errp);
 
     case MIG_CMD_POSTCOPY_RUN:
-        ret = loadvm_postcopy_handle_run(mis);
-        if (ret < 0) {
-            error_setg(errp, "Failed to load device state command: %d", ret);
-        }
-        return ret;
+        return loadvm_postcopy_handle_run(mis, errp);
 
     case MIG_CMD_POSTCOPY_RAM_DISCARD:
         ret = loadvm_postcopy_ram_handle_discard(mis, len);