Multi-tenant DaaS Offering
You can in fact deliver a DaaS offering including XP/Win7/Win8 while leveraging multi-tenancy at the management, brokering, directory services, networking and storage layers. It is true however, that Windows client operating systems are not covered by SPLA and require dedicated hardware at the hypervisor layer (not storage). This is explicitly covered on page 3 of the following FAQ:
While this requirement is nonsensical and only makes money for the hardware providers (not Microsoft), there are a number of cloud service providers doing this today including Dell, Navisite, Dimension Data, Fujitsu and others that do offer full Win client OS machines as a DaaS service.
From digging deeper into the articles being published on this, it seems that the XenDesktop on Azure is really creating the 1:1 user to WinServer "skinned" desktop that does not require the dedicated hardware and can be covered under SPLA licensing.