hw/vfio/platform: change interrupt/unmask fields into pointer

unmask EventNotifier might not be initialized in case of edge
sensitive irq. Using EventNotifier pointers make life simpler to
handle the edge-sensitive irqfd setup.

Signed-off-by: Eric Auger <eric.auger@linaro.org>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
