fw_cfg: unbreak migration compatibility for 2.4 and earlier machines

fw_cfg: unbreak migration compatibility for 2.4 and earlier machines

When I reviewed Marc's fw_cfg DMA patches, I completely missed that the
way we set dma_enabled would break migration.

Gerd explained the right way (see reference below): dma_enabled should be
set to true by default, and only true->false transitions should be
possible:

- when the user requests that with

    -global fw_cfg_mem.dma_enabled=off

  or

   -global fw_cfg_io.dma_enabled=off

  as appropriate for the platform,

- when HW_COMPAT_2_4 dictates it,

- when board code initializes fw_cfg without requesting DMA support.

Cc: Marc Marí <[email protected]>
Cc: Gerd Hoffmann <[email protected]>
Cc: Alexandre DERUMIER <[email protected]>
Cc: [email protected]
Ref: http://thread.gmane.org/gmane.comp.emulators.qemu/390272/focus=391042
Ref: https://bugs.launchpad.net/qemu/+bug/1536487
Suggested-by: Gerd Hoffmann <[email protected]>
Signed-off-by: Laszlo Ersek <[email protected]>
Message-id: 1455823860[email protected]
Signed-off-by: Gerd Hoffmann <[email protected]>