stubs: Rename qmp_memory_device.c to memory_device.c

We want to place non-qmp stubs in there, so let's rename it. While at
it, put it into the MAINTAINERS file under "Memory devices".

Message-ID: <20230926185738.277351-8-david@redhat.com>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

stubs/meson.build

@@ -32,7 +32,7 @@ stub_ss.add(files('monitor.c'))
 stub_ss.add(files('monitor-core.c'))
 stub_ss.add(files('physmem.c'))
 stub_ss.add(files('qemu-timer-notify-cb.c'))
-stub_ss.add(files('qmp_memory_device.c'))
+stub_ss.add(files('memory_device.c'))
 stub_ss.add(files('qmp-command-available.c'))
 stub_ss.add(files('qmp-quit.c'))
 stub_ss.add(files('qtest.c'))