ami: initial support for building HVM AMIs along side PV AMIs.

This just updates the build and test scripts. The copy and upload
scripts still need work to handle the new set of AMIs.
ami: support HVM images in copy script
ami: support HVM images in uploaded .txt files
Merge pull request #292 from marineam/hvm

ami: support for building HVM AMIs along side PV AMIs.