xen: events: remove use of nr_irqs as upper bound on number of pirqs

There isn't really much relationship between the two, other than
nr_irqs often being the larger of the two.

Allows us to remove a nr_irqs sized array, the only users of this
array are MSI setup and restore, neither of which are particularly
performance critical.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>