Even through best efforts at keeping it small, including the ZFS tools
in the initramfs seems to have exhausted the small overhead we had left
- give it a bit more RAM to compensate.
Also disable the ZFS ARC, as it's no use in such a memory constrained
environment, and we cache on the QEMU/rust layer anyway.
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>