i386/kvm: hv-stimer requires hv-time and hv-synic

Synthetic timers operate in hv-time time and Windows won't use these
without SynIC.

Add .dependencies field to kvm_hyperv_properties[] and a generic mechanism
to check dependencies between features.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20190517141924.19024-7-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>