From 1 February 2016, organisations with Windows Server licenses with SA can make use of the Azure Hybrid Use Benefit.
Called “HUB” for short, this brand new benefit means that you can procure an Azure virtual machine without Windows Server and can then assign an existing Windows Server licence to it.
Even more interestingly, if you have Datacenter licences then they can be assigned to on-premises AND Azure virtual machines, while you need to make the choice of either on-premises OR Azure virtual machines for Standard. A single Processor-based licence will allow you to use Windows Server on up to 16 cores in Azure.
Get the full wording on page 50 of the February 2016 Product Terms.