Unify Xilinx platforms into a single class, support more devices

This merges existing code, and also adds support for:

- Virtex, Virtex E (also known as Spartan 2, Spartan 2E)
- Virtex 2, Virtex 2 Pro
- Spartan 3, Spartan 3E (in addition to existing Spartan 3A, Spartan 3A
  DSP support)
- Virtex 4
- Virtex 5
- Virtex 6
- ISE synthesis for Series 7

Fixes #552.