The behavior for the qemu balloon device has changed. Formerly, a virtio
balloon device was provided by default. Now, '-balloon virtio' must be
specified on the command line to enable it. This patch causes libvirt to
add '-balloon virtio' to the command line whenever the -balloon option is
available.
* src/qemu/qemu_conf.c src/qemu/qemu_conf.h: check for the new flag and
add "-baloon vitio" to qemu command when needed
* tests/qemuhelptest.c: add the new flag for detection