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>

include/hw/ppc/spapr.h

@@ -62,6 +62,9 @@ typedef enum {
 /* Vector Scalar Extensions */
 #define SPAPR_CAP_VSX               0x0000000000000002ULL
 
+/* Decimal Floating Point */
+#define SPAPR_CAP_DFP               0x0000000000000004ULL
+
 typedef struct sPAPRCapabilities sPAPRCapabilities;
 struct sPAPRCapabilities {
     uint64_t mask;