xen/smp: Introduce a common structure to contain the IRQ name and interrupt line.

This patch adds a new structure to contain the common two things
that each of the per-cpu interrupts need:
 - an interrupt number,
 - and the name of the interrupt (to be added in 'xen/smp: Don't leak
   interrupt name when offlining').

This allows us to carry the tuple of the per-cpu interrupt data structure
and expand it as we need in the future.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>