migration: Fix !replay_can_snapshot() error handling

Calling error_report() in a function that takes an Error ** argument
is suspicious.  save_snapshot() and load_snapshot() do that, and then
fail without setting an error.  Wrong.  The HMP commands survive this
unscathed, since hmp_handle_error() does nothing when no error has
been set.  Callers main() (on behalf of -loadvm) and
replay_vmstate_init() crash, but I'm not sure the error is possible
there.

Screwed up when commit 377b21cc (v2.12.0) added incorrect error
handling right next to correct examples.  Fix by calling error_setg()
instead of error_report().

Fixes: 377b21cc
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20181017082702.5581-13-armbru@redhat.com>

migration/savevm.c

@@ -2414,8 +2414,8 @@ int save_snapshot(const char *name, Error **errp)
     AioContext *aio_context;
 
     if (!replay_can_snapshot()) {
-        error_report("Record/replay does not allow making snapshot "
-                     "right now. Try once more later.");
+        error_setg(errp, "Record/replay does not allow making snapshot "
+                   "right now. Try once more later.");
         return ret;
     }
 
@@ -2611,8 +2611,8 @@ int load_snapshot(const char *name, Error **errp)
     MigrationIncomingState *mis = migration_incoming_get_current();
 
     if (!replay_can_snapshot()) {
-        error_report("Record/replay does not allow loading snapshot "
-                     "right now. Try once more later.");
+        error_setg(errp, "Record/replay does not allow loading snapshot "
+                   "right now. Try once more later.");
         return -EINVAL;
     }