migration/colo: Improve an x-colo-lost-heartbeat error message

The QERR_ macros are leftovers from the days of "rich" error objects.
We've been trying to reduce their remaining use.

Get rid of a use of QERR_FEATURE_DISABLED, and improve the somewhat
imprecise error message

    (qemu) x_colo_lost_heartbeat
    Error: The feature 'colo' is not enabled

to

    Error: VM is not in COLO mode

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230207075115.1525-12-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>

migration/colo-failover.c

@@ -17,7 +17,6 @@
 #include "migration.h"
 #include "qapi/error.h"
 #include "qapi/qapi-commands-migration.h"
-#include "qapi/qmp/qerror.h"
 #include "qemu/error-report.h"
 #include "trace.h"
 
@@ -78,7 +77,7 @@ FailoverStatus failover_get_state(void)
 void qmp_x_colo_lost_heartbeat(Error **errp)
 {
     if (get_colo_mode() == COLO_MODE_NONE) {
-        error_setg(errp, QERR_FEATURE_DISABLED, "colo");
+        error_setg(errp, "VM is not in COLO mode");
         return;
     }