spapr: Handle Decimal Floating Point (DFP) as an optional capability

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>