KVM: arm64: Exclude mdcr_el2_host from kvm_vcpu_arch

As for the rest of the host debug state, the host copy of mdcr_el2
has little to do in the vcpu, and is better placed in the host_data
structure.

Reviewed-by : Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>