xen/events: Add WARN_ON when quick lookup found invalid type.

All of the bind_XYZ_to_irq do a quick lookup to see if the
event exists. And if it does, then the initialized IRQ number
is returned instead of initializing a new IRQ number.

This patch adds an extra logic to check that the type returned
is proper one and that there is an IRQ handler setup for it.

This patch has the benefit of being able to find drivers that
are doing something naught.

[v1: Enhanced based on Stefano's review]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>