We plan to virtualize SQL Server 2014 via Azure. What licensing requirements do we need to consider in this scenario?

There are two possibilities for the depicted scenario:

1. The deployment of Virtual Machines can be done with Microsoft Azure, where a per-minute rate for the use of the software is due, depending on the virtualized software. For this particular scenario you could obtain a SQL Server image from the Marketplace and pay per-minute for the corresponding use of the software.

2. If you have active Software Assurance (SA), you can upload and deploy your own SQL Server license in Azure free of charge. This is a benefit of the Software Assurance called License Mobility.

Source:

Official Microsoft Website for licensing of Azure Virtual Machines as of 2017-03-12 (https://azure.microsoft.com/en-us/pricing/licensing-faq/)