memory: Move owner-less MemoryRegions to /machine/unattached

memory: Move owner-less MemoryRegions to /machine/unattached

This cleans up the official /machine namespace. In particular
/machine/system[0] and /machine/io[0], as well as entries with
non-sanitized node names such as "/machine/qemu extended regs[0]".

The actual MemoryRegion names remain unchanged.

Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>