pc-dimm: error out if memory hotplug is not enabled

fixes QEMU abort in case it's started without memory
hotplug enabled.

as result of fix it will print following messages:
"
-device pc-dimm,id=d1,memdev=m1: memory hotplug is not enabled, enable it on startup
-device pc-dimm,id=d1,memdev=m1: Device 'pc-dimm' could not be initialized
"

Also fixup assert condition to detect hotplug address
space overflow.
Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
Reported-by: NHu Tao <hutao@cn.fujitsu.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/mem/pc-dimm.c

@@ -146,7 +146,13 @@ uint64_t pc_dimm_get_free_addr(uint64_t address_space_start,
     uint64_t new_addr, ret = 0;
     uint64_t address_space_end = address_space_start + address_space_size;
 
-    assert(address_space_end > address_space_size);
+    if (!address_space_size) {
+        error_setg(errp, "memory hotplug is not enabled, "
+                         "please add maxmem option");
+        goto out;
+    }
+
+    assert(address_space_end > address_space_start);
     object_child_foreach(qdev_get_machine(), pc_dimm_built_list, &list);
 
     if (hint) {