KVM: x86: hyper-v: disallow configuring SynIC timers with no SynIC

Hyper-V Synthetic timers require SynIC but we don't seem to check that
upon HV_X64_MSR_STIMER[X]_CONFIG/HV_X64_MSR_STIMER0_COUNT writes. Make
the behavior match synic_set_msr().

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20200924145757.1035782-3-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>