Old kernels use very small buffers to transfer files between host and guest
and qemu will warn if that is the case. Bump the default to 1MiB.
fix booting v4.9 kernel
cgroupfs on 4.9 doesn't support one of the option and returns EINVAL. Don't
mount it from init, users can use a setup command to achieve the same.
Add 4.9 to CI.
use 1MiB msize to mount 9pfs
fix booting v4.9 kernel