The global variable is not necessary because we can check
cpu->hyperv_time directly.

We just need to ensure cpu->hyperv_time will be cleared if the
feature is not really being exposed to the guest due to missing
KVM_CAP_HYPERV_TIME capability.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>