xen: events: do not workaround too-small nr_irqs

With the introduction of e7bcecb7 "genirq: Make nr_irqs runtime expandable"
nr_irqs can grow as necessary to accommodate our allocation requests.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>