Simplify logic for hotplug notification, by tracking state of the
logical interrupt condition.  We then simply use this variable to make
the interrupt decision, according to spec.

API is made cleaner as we no longer force users to pass in
old slot control value.

Includes fixes by Isaku Yamahata.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NIsaku Yamahata <yamahata@valinux.co.jp>