replace d->nr_pirqs sized arrays with radix tree

replace d->nr_pirqs sized arrays with radix tree

With this it is questionable whether retaining struct domain's
nr_pirqs is actually necessary - the value now only serves for bounds
checking, and this boundary could easily be nr_irqs.

Another thing to consider is whether it's worth storing the pirq
number in struct pirq, to avoid passing the number and a pointer to
quite a number of functions.

Note that ia64, the build of which is broken currently anyway, is only
partially fixed up.

Signed-off-by: Jan Beulich <jbeulich@novell.com>