hw/display/sm501: Don't use vmstate_register_ram_global()

hw/display/sm501: Don't use vmstate_register_ram_global()

The sm501 device uses vmstate_register_ram_global() to register its
memory region for migration.  This means it gets a name that is
assumed to be global to the whole system, which in turn means that if
you create two of the device we assert because of the duplication:

qemu-system-ppc -device sm501 -device sm501
RAMBlock "sm501.local" already registered, abort!
Aborted (core dumped)

Changing this to just use memory_region_init_ram()'s automatic
registration of the memory region with a device-local name fixes
this.  The downside is that it breaks migration compatibility, but
luckily we only added migration support to this device in the 2.10
release cycle so we haven't released a QEMU version with the broken
implementation.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-id: 1500309462-12792-1-git-send-email-peter.maydell@linaro.org