numa: check for busy memory backend

Specifying the same memory backend twice leads to an assert:

./x86_64-softmmu/qemu-system-x86_64 -m 512M -enable-kvm -object
memory-backend-ram,size=256M,id=ram0 -numa node,nodeid=0,memdev=ram0
-numa node,nodeid=1,memdev=ram0
qemu-system-x86_64: /scm/qemu/memory.c:1506:
memory_region_add_subregion_common: Assertion `!subregion->container'
failed.
Aborted (core dumped)

Detect and exit with an error message instead.
Reviewed-by: NIgor Mammedov <imammedo@redhat.com>
Signed-off-by: NHu Tao <hutao@cn.fujitsu.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

numa.c

@@ -301,6 +301,14 @@ void memory_region_allocate_system_memory(MemoryRegion *mr, Object *owner,
             exit(1);
         }
 
+        if (memory_region_is_mapped(seg)) {
+            char *path = object_get_canonical_path_component(OBJECT(backend));
+            error_report("memory backend %s is used multiple times. Each "
+                         "-numa option must use a different memdev value.",
+                         path);
+            exit(1);
+        }
+
         memory_region_add_subregion(mr, addr, seg);
         vmstate_register_ram_global(seg);
         addr += size;