virtio-balloon: add hugetlb page allocation counts

virtio-balloon: add hugetlb page allocation counts

qemu should read and report hugetlb page allocation
counts exported in the following kernel patch:

    commit 4c3ca37c4a4394978fd0f005625f6064ed2b9a64
    Author: Jonathan Helman <jonathan.helman@oracle.com>
    Date:   Mon Mar 19 11:00:35 2018 -0700

    virtio_balloon: export hugetlb page allocation counts

    Export the number of successful and failed hugetlb page
    allocations via the virtio balloon driver. These 2 counts
    come directly from the vm_events HTLB_BUDDY_PGALLOC and
    HTLB_BUDDY_PGALLOC_FAIL.

Signed-off-by: Jonathan Helman <jonathan.helman@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>