memory-device: Unify enabled vs. supported error messages

Let's unify the error messages, such that we can simply stop allocating
ms->device_memory if the size would be 0 (and there are no memory
devices ever).

The case of "not supported by the machine" should barely pop up either
way: if the machine doesn't support memory devices, it usually doesn't
call the pre_plug handler ...

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20230623124553.400585-2-david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

hw/mem/memory-device.c

@@ -104,15 +104,10 @@ static uint64_t memory_device_get_free_addr(MachineState *ms,
     GSList *list = NULL, *item;
     Range as, new = range_empty;
 
-    if (!ms->device_memory) {
-        error_setg(errp, "memory devices (e.g. for memory hotplug) are not "
-                         "supported by the machine");
-        return 0;
-    }
-
-    if (!memory_region_size(&ms->device_memory->mr)) {
-        error_setg(errp, "memory devices (e.g. for memory hotplug) are not "
-                         "enabled, please specify the maxmem option");
+    if (!ms->device_memory || !memory_region_size(&ms->device_memory->mr)) {
+        error_setg(errp, "the configuration is not prepared for memory devices"
+                         " (e.g., for memory hotplug), consider specifying the"
+                         " maxmem option");
         return 0;
     }
     range_init_nofail(&as, ms->device_memory->base,