Firecracker micro-vm fails to boot. It turns out that the following
configs must be enabled and set to built-in:

- CONFIG_VIRTIO_MMIO
- CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES
Signed-off-by: NCaspar Zhang <caspar@linux.alibaba.com>
Tested-by: NEryu Guan <eguan@linux.alibaba.com>