In fact, we seem to lack a generic memory device in the naming spec.

A generic memory icon shouldn't be used for this purpose. It dilutes the metaphor, by using it for something stored in memory, rather than the memory itself. That would be like using a hard disk icon to respresent files on the disk. Given that we're trying to represent virtual disks here, perhaps the icon to use would be either drive-harddisk or media-flash. Both of these icons are I think more suited to the representation. In fact, OS X uses an icon that looks like an external hard disk for disk images.