tests: add simple balloon functional testcase

Change-Id: Ie8140fe1c754d9d4026c503a19420e6552a3f4fe
Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12820
Reviewed-by: Dan Kenigsberg <danken@redhat.com>