spapr_vty: lookup should only return valid VTY objects

If a guest passes the reg property of a valid VIO object that is not a VTY
to either H_GET_TERM_CHAR or H_PUT_TERM_CHAR, QEMU hits a dynamic cast
assertion and aborts.

PAPR+ says "Hypervisor checks the termno parameter for validity against the
Vterm IOA unit addresses assigned to the partition, else return H_Parameter."

This patch adds a type check to ensure vty_lookup() either returns a pointer
to a valid VTY object or NULL.  H_GET_TERM_CHAR and H_PUT_TERM_CHAR will
now return H_PARAMETER to the guest instead of crashing.

The patch has no effect on the reg == 0 hack used to implement the RTAS call
display-character.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>

hw/char/spapr_vty.c

@@ -228,6 +228,10 @@ VIOsPAPRDevice *vty_lookup(sPAPRMachineState *spapr, target_ulong reg)
         return spapr_vty_get_default(spapr->vio_bus);
     }
 
+    if (!object_dynamic_cast(OBJECT(sdev), TYPE_VIO_SPAPR_VTY_DEVICE)) {
+        return NULL;
+    }
+
     return sdev;
 }