spapr_pci: Advertise MSI quota

Hotplug of multiple disks fails due to MSI vector quota check.
Number of MSI vectors default to 8 allowing only 4 devices.
This happens on RHEL6.5 guest. RHEL7 and SLES11 guests fallback
to INTX.

One way to workaround the issue is to increase total MSIs,
so that MSI quota check allows us to hotplug multiple disks.

This sets the quota to the maximum number of interupts XICS has
which is 1024 now (XICS_IRQS). This moves XICS_IRQS from spapr.c
to xics.h for wider visibility.

Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
[aik: put XICS_IRQS=1024 instead of 64i, fixed endianness and size]
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>