arm64: KVM: export demux regids as KVM_REG_ARM64

I suspect this is a -ECUTPASTE fault from the initial implementation. If
we don't declare the register ID to be KVM_REG_ARM64 the KVM_GET_ONE_REG
implementation kvm_arm_get_reg() returns -EINVAL and hilarity ensues.

The kvm/api.txt document describes all arm64 registers as starting with
0x60xx... (i.e KVM_REG_ARM64).

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>