virtio-devices: balloon: Enable use with confidential VMs

Following the pattern used by the existing virtio devices make the
balloon device work with confidential VMs (e.g. SEV-SNP). This requires
advertising the VIRTIO_F_ACCESS_PLATFORM feature. Do not expose this to
the user as a controllable option and instead only enable in on the
"force" case.

Signed-off-by: Rob Bradford <rbradford@meta.com>

fuzz/fuzz_targets/balloon.rs

@@ -49,6 +49,7 @@ fuzz_target!(|bytes: &[u8]| -> Corpus {
         BALLOON_SIZE,
         true,
         true,
+        false,
         SeccompAction::Allow,
         EventFd::new(EFD_NONBLOCK).unwrap(),
         None,