arm64: KVM: kvm_arch and kvm_vcpu_arch definitions

Provide the architecture dependent structures for VM and
vcpu abstractions.

Reviewed-by: Christopher Covington <cov@codeaurora.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>