memory: Assert that memory_region_init_rom_device() ops aren't NULL

It doesn't make sense to pass a NULL ops argument to
memory_region_init_rom_device(), because the effect will
be that if the guest tries to write to the memory region
then QEMU will segfault. Catch the bug earlier by sanity
checking the arguments to this function, and remove the
misleading documentation that suggests that passing NULL
might be sensible.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1467122287-24974-4-git-send-email-peter.maydell@linaro.org
