virttest: Add balloon device in qemu_vm

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
virttest: Correct a wrong docstring in qemu_vm

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1828 from ypu/balloon-update

Add balloon device using qemu_device functions