Add inception support: KVM-enabled kernel and /dev/kvm creation

[ejc3]

Summary

Adds inception support for running fcvm inside fcvm (nested virtualization).

• kernel/build.sh: Build script for custom kernel with CONFIG_KVM=y + CONFIG_FUSE_FS=y
• kernel/inception.conf: Kernel config fragment for inception
• fc-agent: Create /dev/kvm device at boot via mknod (10:232)
• --kernel flag: Specify custom kernel path (e.g., for inception)
• local_path in config: Alternative to URL for kernel in rootfs-config.toml
• test_kvm.rs: Verify /dev/kvm works in VM and privileged container

Key Design

Content-addressed caching: The inception kernel name includes the SHA of build.sh + inception.conf, so changes automatically trigger rebuilds.

fc-agent creates /dev/kvm at boot. This is a no-op if the kernel doesn't have CONFIG_KVM (the mknod succeeds but device won't work). With inception kernel, mknod creates a functional device.

Test Plan

• make test-root FILTER=kvm - passed
• fc-agent created /dev/kvm (10:232)
• /dev/kvm accessible from VM (test -r && test -w)
• /dev/kvm accessible from privileged container

Usage

# Build inception kernel (first time only, ~5 min)
./kernel/build.sh

# Run VM with inception kernel
sudo fcvm podman run --name my-vm --network bridged \
    --kernel /mnt/fcvm-btrfs/kernels/vmlinux-6.12.10-*.bin \
    --privileged nginx:alpine