Support host/guest notifiers in virtio-pci.
The last one only with kvm, that's okay
because vhost relies on kvm anyway.

Note on kvm usage: kvm ioeventfd API
is implemented on non-kvm systems as well,
this is the reason we don't need if (kvm_enabled())
around it.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>