Fix ballooning rules for computing the minimum available memory

Change-Id: Ie416db6580462bbd16e80bea0a4e339656eccb0f
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1025845
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20849
Reviewed-by: Doron Fediuck <dfediuck@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>