exec: change default exception_index value for migration to -1

In QEMU 2.2 the exception_index value was added to the migration stream
through a subsection.  The default was set to 0, which is wrong and
should have been -1.

However, 2.2 does not have commit e511b4d (cpu-exec: reset exception_index
correctly, 2014-11-26), hence in 2.2 the exception_index is never used
and is set to -1 on the next call to cpu_exec.  So we can change the
migration stream to make the default -1.  The effects are:

- 2.2.1 -> 2.2.0: cpu->exception_index set incorrectly to 0 if it
were -1 on the source; then reset to -1 in cpu_exec.  This is TCG
only; KVM does not use exception_index.

- 2.2.0 -> 2.2.1: cpu->exception_index set incorrectly to -1 if it
were 0 on the source; but it would be reset to -1 in cpu_exec anyway.
This is TCG only; KVM does not use exception_index.

- 2.2.1 -> 2.1: two bugs fixed: 1) can migrate backwards if
cpu->exception_index is set to -1; 2) should not migrate backwards
(but 2.2.0 allows it) if cpu->exception_index is set to 0

- 2.2.0 -> 2.3.0: 2.2.0 will send the subsection unnecessarily if
exception_index is -1, but that is not a problem.  2.3.0 will set
cpu->exception_index to -1 if it is 0 on the source, but this would
be anyway a problem for 2.2.0 -> 2.2.x migration (due to lack of
commit e511b4d in 2.2.x) so we can ignore it

- 2.2.1 -> 2.3.0: everything works.

In addition, play it safe and never send the subsection unless TCG
is in use.  KVM does not use exception_index (PPC KVM stores values
in it for use in the subsequent call to ppc_cpu_do_interrupt, but
does not need it as soon as kvm_handle_debug returns).  Xen and
qtest do not run any code for the CPU at all.

Reported-by: Igor Mammedov <imammedo@redhat.com>
Tested-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Tested-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1418989994-17244-3-git-send-email-pbonzini@redhat.com
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
(cherry picked from commit adee642)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

exec.c

@@ -434,7 +434,7 @@ static int cpu_common_pre_load(void *opaque)
 {
     CPUState *cpu = opaque;
 
-    cpu->exception_index = 0;
+    cpu->exception_index = -1;
 
     return 0;
 }
@@ -443,7 +443,7 @@ static bool cpu_common_exception_index_needed(void *opaque)
 {
     CPUState *cpu = opaque;
 
-    return cpu->exception_index != 0;
+    return tcg_enabled() && cpu->exception_index != -1;
 }
 
 static const VMStateDescription vmstate_cpu_common_exception_index = {