[core-kit] [kernel] We need to turn on MEMCG_V1 support in sys-kernel/debian-sources for our old qemu

[cuantar]

Our aged QEMU expects CONFIG_MEMCG_V1=y, but the debian-sources kernel config does not turn it on. The issue can be fixed by enabling that option in the ebuild, using the function that exists for that purpose. The symptom is that libvirt VMs don't start, because the expected memgroup-v1 facility doesn't exist, so there isn't a dir for it where there should be.

Now that things are working again, the exact error message is lost (it doesn't get logged), but it's something like "No such file or directory" for a path like /sys/fs/cgroup/memory/machine/qemu-8-v0.libvirt-qemu/memory.use_hierarchy.

(It seems CONFIG_MEMCG_V1 is a new kconfig option in linux-6.11)