virtio: backend virtqueue notifier masking
some backends (notably vhost) can mask events
at their source in a way that is more efficient
than masking through kvm.
Specifically
- masking in kvm uses rcu write side so it has high latency
- in kvm on unmask we always send an interrupt
masking at source does not have these issues.
Add such support in virtio.h and use in virtio-pci.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
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