Enable ballooning in x86_64

Bug: 392795840
Test: n/a
Change-Id: Id3fcb066cbe844e6e4ae5c815370877512e92703
diff --git a/build/debian/vm_config.json.x86_64 b/build/debian/vm_config.json.x86_64
index 496e684..c34a0f2 100644
--- a/build/debian/vm_config.json.x86_64
+++ b/build/debian/vm_config.json.x86_64
@@ -43,5 +43,6 @@
     "debuggable": true,
     "console_out": true,
     "console_input_device": "ttyS0",
-    "network": true
+    "network": true,
+    "auto_memory_balloon": true
 }