hw/net/stellaris_enet: Convert to vmstate

Convert this device to use vmstate for its save/load, including
providing a post_load function that sanitizes inbound data to
avoid possible buffer overflows if it is malicious.

The sanitizing fixes CVE-2013-4532 (though nobody should be
relying on the security properties of most of the unmaintained
ARM board models anyway, and migration doesn't actually
work on this board due to issues in other device models).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
