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 <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Message-Id: <20190214043916[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>