Do I need a license to design IP cores with AXI interfaces?
The AMBA specification (which includes all of the AXI-3, AXI-4, ACP, AXI Stream protocol) is available for license from ARM for no cost.  To get the license, you need to create an account at ARM.com (no cost), and download the license ( no cost, but requires you to click through a legal agreement).

This license enables you to design products that uses these buses, sell them, buy them.  You can not transfer your license to some else (so perhaps you are using the Xilinx license now as a developer (grey area), but if you want to sell the product you've developed, you should click through and get a license.  (as a side effect, you get the actual specifications from ARM, which will be useful in making sure your IPs are correct!)  

One restriction: Under this license, you can build and sell products that include an AMBA bus and a CPU, but the CPU cannot be code-compatible with ARM, unless it includes a paid for ARM license.

What I say where is just my opinion from reading the ARM license agreement.  Go get your own license agreement from www.arm.com and verify this for yourself, with all of the legal concurrence you can afford.