KVM: selftests: Fix typo in vgic_init test

When iterating over vCPUs, invoke access_v3_redist_reg() on the "current"
vCPU instead of vCPU0, which is presumably what was intended by iterating
over all vCPUs.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>