IRQ: allocate CPU masks dynamically

IRQ: allocate CPU masks dynamically

This includes delaying the initialization of dynamically created IRQs
until their actual first use and some further elimination of uses of
struct irq_cfg.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>