balloon: align val in virtio_balloon_stat

we have:
struct virtio_balloon_stat

{ #define VIRTIO_BALLOON_S_SWAP_IN 0 #define VIRTIO_BALLOON_S_SWAP_OUT 1 #define VIRTIO_BALLOON_S_MAJFLT 2 #define VIRTIO_BALLOON_S_MINFLT 3 #define VIRTIO_BALLOON_S_MEMFREE 4 #define VIRTIO_BALLOON_S_MEMTOT 5 u16 tag; u64 val; }

_attribute_((packed));

val is 8 bytes but aligned at a 2 byte boundary.
this is ugly and unusual, and requires implementations
to use packed structures for access, which is error-prone.