KVM: ARM: Access all registers via KVM_GET_ONE_REG/KVM_SET_ONE_REG.

We use the offset within the structure (which, conveniently, is used
internally as well) to supply unique ids for the core registers.

This makes it trivially extensible by appending fields to the struct.

Signed-off-by: Rusty Russell <rusty.russell@linaro.org>