Excerpts from Microsoft Licensing Guide
How to License Individual Virtual Machines Using the Per Core Licensing Model
*Similar to the Per Core licensing model in physical OSEs, all virtual cores (v-cores) supporting virtual OSEs that
are running instances of SQL Server 2012 software must be licensed accordingly.
To license individual VMs using the Per Core model, customers must purchase a core license for each v-core (or virtual processor, virtual CPU, virtual thread) allocated to the VM, subject to a four core license minimum
per VM. For licensing purposes, a v-core maps to a hardware thread. When licensing individual VMs, core factors do not apply.
Note: Licensing individual VMs is the only licensing option available for SQL Server 2012 Standard Edition
customers who are running the software in a virtualized environment under the Per Core model.*
Licensing is tricky, I would encourage you contact Microsoft to get true answer
You should call 1-800-426-9400, Monday through Friday, 6:00 A.M. to 6:00 P.M. (Pacific Time) to speak directly to a Microsoft licensing specialist, and you can get more detail information from them. Outside North Ameriac you can use the Guide to Worldwide Microsoft Licensing Sites to find contact information in your regional MS location.