virtio_balloon: add array of stat names

Jason Wang points out that it's very hard for users to build an array of
stat names. The naive thing is to use VIRTIO_BALLOON_S_NR but that
breaks if we add more stats - as done e.g. recently by commit 6c64fe7f
("virtio_balloon: export hugetlb page allocation counts").

Let's add an array of reasonably readable names.

Fixes: 6c64fe7f ("virtio_balloon: export hugetlb page allocation counts")
Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Jonathan Helman <jonathan.helman@oracle.com>