virtio-balloon: Rework ballon_page() interface

virtio-balloon: Rework ballon_page() interface

This replaces the balloon_page() internal interface with
ballon_inflate_page(), with a slightly different interface.  The new
interface will make future alterations simpler.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20190214043916.22128-4-david@gibson.dropbear.id.au>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit e9550234d79ddb69b01721d8cb197edc0a14a245)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>