hw/virtio-net: disable multiqueue by default

The new multiqueue feature adds fields to the virtio device config, which
breaks Windows guests. Disable the feature by default until the Windows
drivers are fixed.

Signed-off-by: Jesse Larrew <jlarrew@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>