vdsm: move balloon logic out of buildConfDevices

Encapsulates empty balloon creation logic outside of
buildConfDevices to improve code readability

Change-Id: I6d77c1bbd7bdf760c8dda35d0ce6b70af51326bd
Signed-off-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19321
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>