KVM: s390: Use kvm_get_vcpu() instead of open-coded access

As we are about to change the way vcpus are allocated, mandate
the use of kvm_get_vcpu() instead of open-coding the access.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Message-Id: <20211116160403.4074052-4-maz@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>