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.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1034969
Change-Id: I2a8d19f7fe66bb98d97718da34ec2bad6f7987f2
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21765
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Tested-by: Yaniv Bronhaim <ybronhei@redhat.com>