Detect on reconnect to a running qemu VM whether the alias of a
hotpluggable memory device (dimm) does not match the dimm slot number
where it's connected to. This is necessary as qemu is actually
considering the alias as machine ABI used to connect the backend object
to the dimm device.

This will require us to keep them consistent so that we can reliably
restore them on migration. In some situations it was currently possible
to create a mismatched configuration and qemu would refuse to restore
the migration stream.

To avoid breaking existing VMs we'll need to keep the old algorithm
though.