virt: move balloon device to vmdevices.py

No real code changes

Change-Id: I16b04a84e8bbdefe9033f96c2a3b89d1ea90ba8c
Signed-off-by: Martin Polednik <mpolednik@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34309
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>