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/microfuchsia/microfuchsiad/src/instance_starter.rs b/microfuchsia/microfuchsiad/src/instance_starter.rs
index 8216039..e3c4e8d 100644
--- a/microfuchsia/microfuchsiad/src/instance_starter.rs
+++ b/microfuchsia/microfuchsiad/src/instance_starter.rs
@@ -86,6 +86,7 @@
             platformVersion: "1.0.0".into(),
             #[cfg(enable_console)]
             consoleInputDevice: Some("ttyS0".into()),
+            balloon: true,
             ..Default::default()
         });
         let vm_instance = VmInstance::create(