Reorder limit checks in grow quest

balloon_min is a computed value that can (and will) go over
balloon_max. When the controller tries to use it VDSM reports
exception.

Reorder the checks so the balloon_max constant check is the latest
one to limit the value.

Change-Id: I3e542b5d1c165e32b8555e5510660ced136c6ab9
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21726
Reviewed-by: Adam Litke <alitke@redhat.com>