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 2293c27f (i2c: implement broadcast write)

Fix it by dropping 'broadcast' VMState introduced by 2293c27f 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>
