spapr: don't advertise radix GTSE if max-compat-cpu < power9
    
    On a POWER9 host, if a guest runs in pre POWER9 compat mode, it necessarily
    uses the hash MMU mode. In this case, we shouldn't advertise radix GTSE in
    the ibm,arch-vec-5-platform-support DT property as the current code does.
    The first reason is that it doesn't make sense, and the second one is that
    causes the CAS-negotiated options subsection to be migrated. This breaks
    backward migration to QEMU 2.7 and older versions on POWER8 hosts:
    
    qemu-system-ppc64: error while loading state for instance 0x0 of device
     'spapr'
    qemu-system-ppc64: load of migration failed: No such file or directory
    
    This patch hence initialize CPUs a bit earlier so that we can check the
    requested compat mode, and don't set OV5_MMU_RADIX_GTSE for power8 and
    older.
    
    Signed-off-by: Greg Kurz <groug@kaod.org>
    Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
    (cherry picked from commit 0550b1206a91d66051a21441a02c4ff126b531fe)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>