hw/vfio/platform: add irq assignment

This patch adds the code requested to assign interrupts to
a guest. The interrupts are mediated through user handled
eventfds only.

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