pc/fwcfg: unbreak migration from qemu-2.5 and qemu-2.6 during firmware boot

pc/fwcfg: unbreak migration from qemu-2.5 and qemu-2.6 during firmware boot

Since 2.7 commit (b2a575a Add optionrom compatible with fw_cfg DMA version)
regressed migration during firmware exection time by
abusing fwcfg.dma_enabled property to decide loading
dma version of option rom AND by mistake disabling DMA
for 2.6 and earlier globally instead of only for option rom.

so 2.6 machine type guest is broken when it already runs
firmware in DMA mode but migrated to qemu-2.7(pc-2.6)
at that time;

a) qemu-2.6:pc2.6 (fwcfg.dma=on,firmware=dma,oprom=ioport)
b) qemu-2.7:pc2.6 (fwcfg.dma=off,firmware=ioport,oprom=ioport)

  to:   a     b
from
a       OK   FAIL
b       OK   OK

So we currently have broken forward migration from
qemu-2.6 to qemu-2.[789] that however could be fixed
for 2.10 by re-enabling DMA for 2.[56] machine types
and allowing dma capable option rom only since 2.7.
As result qemu should end up with:

c) qemu-2.10:pc2.6 (fwcfg.dma=on,firmware=dma,oprom=ioport)

   to:  a     b    c
from
a      OK   FAIL  OK
b      OK   OK    OK
c      OK   FAIL  OK

where forward migration from qemu-2.6 to qemu-2.10 should
work again leaving only qemu-2.[789]:pc-2.6 broken.

Reported-by: Eduardo Habkost <ehabkost@redhat.com>
Analyzed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>