s390x/migration: add comment about floating point migration

commit 46c804def4 ("s390x: move fpu regs into a subsection
of the vmstate") moved the fprs into a subsection and bumped
the version number. This will allow to not transfer fprs in
the future if necessary. Add a comment to mark the return true
as intentional.

CC: Juan Quintela <quintela@redhat.com>
CC: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <1433758884-2997-1-git-send-email-borntraeger@de.ibm.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>

target-s390x/machine.c

@@ -70,6 +70,7 @@ const VMStateDescription vmstate_fpu = {
 
 static inline bool fpu_needed(void *opaque)
 {
+    /* This looks odd, but we might want to NOT transfer fprs in the future */
     return true;
 }