memory: Make memory_region_is_mapped() succeed when mapped via an alias

memory: Make memory_region_is_mapped() succeed when mapped via an alias

memory_region_is_mapped() currently does not return "true" when a memory
region is mapped via an alias.

Assuming we have:
    alias (A0) -> alias (A1) -> region (R0)
Mapping A0 would currently only make memory_region_is_mapped() succeed
on A0, but not on A1 and R0.

Let's fix that by adding a "mapped_via_alias" counter to memory regions and
updating it accordingly when an alias gets (un)mapped.

I am not aware of actual issues, this is rather a cleanup to make it
consistent.

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: <20211102164317.45658-3-david@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>