disable virtio-balloon by default for VirtualMachineRawConfig

So that we don't need to add an LL-NDK API to disable it.

To reduce the impact of this change, I've kept the balloon enabled for
rialto and Trusty even though it is likely not needed.

Test: TH
Bug: 369588412
Change-Id: If22280957d735bd40e86b4d43869a4b70a233cd4
diff --git a/libs/vmconfig/src/lib.rs b/libs/vmconfig/src/lib.rs
index ef932c2..e520f0e 100644
--- a/libs/vmconfig/src/lib.rs
+++ b/libs/vmconfig/src/lib.rs
@@ -133,6 +133,7 @@
                 .collect::<Result<_>>()?,
             consoleInputDevice: self.console_input_device.clone(),
             usbConfig: usb_config,
+            balloon: true,
             ..Default::default()
         })
     }