hw/net/dp8393x: don't make prom region 'nomigrate'

hw/net/dp8393x: don't make prom region 'nomigrate'

Currently we use memory_region_init_rom_nomigrate() to create
the "dp3893x-prom" memory region, and we don't manually register
it with vmstate_register_ram(). This currently means that its
contents are migrated but as a ram block whose name is the empty
string; in future it may mean they are not migrated at all. Use
memory_region_init_ram() instead.

Note that this is a a cross-version migration compatibility break
for the MIPS "magnum" and "pica61" machines.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Aleksandar Markovic <aleksandar.markovic@wavecomp.com>
Message-id: 20180706174309.27110-1-peter.maydell@linaro.org