MAINTAINERS: Add Host Memory Backends section

The hostmem code is closely related to the NUMA code, so I am
willing to handle patches to those files and share the work with
Igor (the original author of that code).

Acked-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>