virttest: Add balloon device in qemu_vm

change from v1:
  - Make all options can be set up by cfg files for furthur test request

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
qemu.tests: Update balloon related cfg files

Update the parameter for add balloon device

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

Signed-off-by: Yiqiao Pu <ypu@redhat.com>