xen/events: only register debug interrupt for 2-level events

xen_debug_interrupt() is specific to 2-level event handling. So don't
register it with fifo event handling being active.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Link: https://lore.kernel.org/r/20201022094907.28560-4-jgross@suse.com
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>