Add support for `KVM_GET_REG_LIST in kvm-ioctls

[dianpopa]

In order to obtain the guest registers that are supported for the KVM_GET_ONE_REG/KVM_SET_ONE_REG ioctl, we need to be able to call into KVM_REG_LIST. See rust-vmm/kvm#104.

[dianpopa]

Feature was merged in rust-vmm. I still need to merge this into https://github.com/firecracker-microvm/kvm-ioctls.

[dianpopa]

Fixed by firecracker-microvm/kvm-ioctls#7.