spapr: Handle Decimal Floating Point (DFP) as an optional capability
    
    Decimal Floating Point has been available on POWER7 and later (server)
    cpus.  However, it can be disabled on the hypervisor, meaning that it's
    not available to guests.
    
    We currently handle this by conditionally advertising DFP support in the
    device tree depending on whether the guest CPU model supports it - which
    can also depend on what's allowed in the host for -cpu host.  That can lead
    to confusion on migration, since host properties are silently affecting
    guest visible properties.
    
    This patch handles it by treating it as an optional capability for the
    pseries machine type.
    
    Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
    Reviewed-by: Greg Kurz <groug@kaod.org>
    (cherry picked from commit 2d1fb9bc8e6e78931d8e1bfeb0ed7a4d223b0480)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>