xen/balloon: Only mark a page as managed when it is released

xen/balloon: Only mark a page as managed when it is released

Only mark a page as managed when it is released back to the allocator.
This ensures that the managed page count does not get falsely increased
when a VM is running. Correspondingly change it so that pages are
marked as unmanaged after getting them from the allocator.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
OraBug: 25497392

(cherry picked from commit 709613ad2b3c9eaeb2a3e24284b7c8feffc17326)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>