target/ppc: always set PPC_MEM_TLBIE in pre 2.8 migration hack
    
    The pseries-2.7 and older machine types require CPUPPCState::insns_flags
    to be strictly equal between source and destination. This checking is
    abusive and breaks migration of KVM guests when the host CPU models
    are different, even if they are compatible enough to allow the guest
    to run transparently. This buggy behaviour was fixed for pseries-2.8
    and we added some hacks to allow backward migration of older machine
    types. These hacks assume that the CPU belongs to the POWER8 family,
    which was true for most KVM based setup we cared about at the time.
    But now POWER9 systems are coming, and backward migration of pre 2.8
    guests running in POWER8 architected mode from a POWER9 host to a
    POWER8 host is broken:
    
    qemu-system-ppc64: error while loading state for instance 0x0 of device
     'cpu'
    qemu-system-ppc64: load of migration failed: Invalid argument
    
    This happens because POWER9 doesn't set PPC_MEM_TLBIE in insns_flags,
    while POWER8 does. Let's force PPC_MEM_TLBIE in the migration hack to
    fix the issue. This is an acceptable hack because these old machine
    types only support CPU models that do set PPC_MEM_TLBIE.
    
    Signed-off-by: Greg Kurz <groug@kaod.org>
    Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
    (cherry picked from commit bce009645b9f1d59195518e35747c8ea30f985f7)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>