Encode a useful name in the memory policy.

The instance name is the (essentially) randomnly generated libvirt
instance-000hex name. It is useless vis-a-vis matching with a policy for any
class of instances.

Instead, encode, the name of the blessed instance. That way, one can build
policies reasoning on properties that should apply uniformly to a generation.

Change-Id: If0c36d886b00e79a27cc0aee7f0246dcb3166541