powerpc/xmon: Add a dump of all XIVE interrupts

Modify the xmon 'dxi' command to query all interrupts if no IRQ number
is specified.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190814154754.23682-4-clg@kaod.org