memory_region_is_mapped() is the wrong check, we actually want to check
whether the backend is already marked mapped.

For example, memory regions mapped via an alias, such as NVDIMMs,
currently don't make memory_region_is_mapped() return "true". As the
machine is initialized before any memory devices (and thereby before
NVDIMMs are initialized), this isn't a fix but merely a cleanup.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20211102164317.45658-2-david@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>