event notifiers are slightly generalized eventfd descriptors. Current
implementation depends on eventfd because vhost is the only user, and
vhost depends on eventfd anyway, but a stub is provided for non-eventfd
case.

We'll be able to further generalize this when another user comes along
and we see how to best do this.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>