spapr: set vsmt to MAX(8, smp_threads)

We ignore silently the value of smp_threads when we set
the default VSMT value, and if smp_threads is greater than VSMT
kernel is going into trouble later.

Fixes: 8904e5a750
("spapr: Adjust default VSMT value for better migration compatibility")

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
(cherry picked from commit 4ad64cbd0c)
Signed-off-by: Greg Kurz <groug@kaod.org>

hw/ppc/spapr.c

@@ -2289,7 +2289,7 @@ static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
          * the value that we'd get with KVM on POWER8, the
          * overwhelmingly common case in production systems.
          */
-        spapr->vsmt = 8;
+        spapr->vsmt = MAX(8, smp_threads);
     }
 
     /* KVM: If necessary, set the SMT mode: */