kvmtool may not be in the mainline kernel source tree, but that doesn't mean it's dead or that it's failed. It had no need to be in the kernel source tree in the first place -- it's a standalone userspace tool that uses the publicly documented and stable KVM syscalls and ioctls, exactly like QEMU. QEMU isn't in the kernel source tree either, and nobody's ever suggested that putting it there would be a good idea...