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: I2a8d19f7fe66bb98d97718da34ec2bad6f7987f2
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21727
Reviewed-by: Adam Litke <alitke@redhat.com>