ppc/xics: Add xics to the monitor "info pic" command

Useful to debug interrupt problems.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[clg: - updated for qemu-2.7
      - added a test on ->irqs as it is not necessarily allocated
        (PHB3_MSI)
      - removed static variable g_xics and replace with a loop on all
        children to find the xics objects.
      - rebased on InterruptStatsProvider interface ]
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
