arm/arm64: kvm: Set vcpu->cpu to -1 on vcpu_put

arm/arm64: kvm: Set vcpu->cpu to -1 on vcpu_put

The arch-generic KVM code expects the cpu field of a vcpu to be -1 if
the vcpu is no longer assigned to a cpu.  This is used for the optimized
make_all_cpus_request path and will be used by the vgic code to check
that no vcpus are running.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
(cherry picked from commit e9b152cb957cb194437f37e79f0f3c9d34fe53d6)
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>