i2c: fix migration regression introduced by broadcast support

QEMU fails migration with following error:

qemu-system-x86_64: Missing section footer for i2c_bus
qemu-system-x86_64: load of migration failed: Invalid argument

when migrating from:
  qemu-system-x86_64-v2.6.0 -m 256M rhel72.img -M pc-i440fx-2.6
to
  qemu-system-x86_64-v2.7.0-rc0 -m 256M rhel72.img -M pc-i440fx-2.6

Regression is added by commit 2293c27 (i2c: implement broadcast write)

Fix it by dropping 'broadcast' VMState introduced by 2293c27 and
reuse broadcast 0x00 address as broadcast flag in bus->saved_address.
Then if there were ongoing broadcast at migration time, set
bus->saved_address to it and at i2c_slave_post_load() time check
for it instead of transfering and using 'broadcast' VMState.

As result of reusing existing saved_address VMState, no compat
glue will be needed to keep forward/backward compatiblity. which
makes fix much less intrusive.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1469623198-177227-1-git-send-email-imammedo@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/i2c/core.c

@@ -17,6 +17,8 @@ struct I2CNode {
     QLIST_ENTRY(I2CNode) next;
 };
 
+#define I2C_BROADCAST 0x00
+
 struct I2CBus
 {
     BusState qbus;
@@ -47,6 +49,8 @@ static void i2c_bus_pre_save(void *opaque)
     if (!QLIST_EMPTY(&bus->current_devs)) {
         if (!bus->broadcast) {
             bus->saved_address = QLIST_FIRST(&bus->current_devs)->elt->address;
+        } else {
+            bus->saved_address = I2C_BROADCAST;
         }
     }
 }
@@ -58,7 +62,6 @@ static const VMStateDescription vmstate_i2c_bus = {
     .pre_save = i2c_bus_pre_save,
     .fields = (VMStateField[]) {
         VMSTATE_UINT8(saved_address, I2CBus),
-        VMSTATE_BOOL(broadcast, I2CBus),
         VMSTATE_END_OF_LIST()
     }
 };
@@ -93,7 +96,7 @@ int i2c_start_transfer(I2CBus *bus, uint8_t address, int recv)
     I2CSlaveClass *sc;
     I2CNode *node;
 
-    if (address == 0x00) {
+    if (address == I2C_BROADCAST) {
         /*
          * This is a broadcast, the current_devs will be all the devices of the
          * bus.
@@ -221,7 +224,8 @@ static int i2c_slave_post_load(void *opaque, int version_id)
     I2CNode *node;
 
     bus = I2C_BUS(qdev_get_parent_bus(DEVICE(dev)));
-    if ((bus->saved_address == dev->address) || (bus->broadcast)) {
+    if ((bus->saved_address == dev->address) ||
+        (bus->saved_address == I2C_BROADCAST)) {
         node = g_malloc(sizeof(struct I2CNode));
         node->elt = dev;
         QLIST_INSERT_HEAD(&bus->current_devs, node, next);