Guest visible data shouldn't change with a simple QEMU upgrade, so use
qemu_hw_version() to ensure it won't change (as long as the machine
class being used has hw_version set).

Cc: Hannes Reinecke <hare@suse.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-block@nongnu.org
Reviewed-by: Hannes Reinecke <hare@suse.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <1446233769-7892-4-git-send-email-ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>