Change xen -> xen-hvm for AWS heavy stemcells
Change xen -> xen-hvm for AWS heavy stemcells

[#134097225](https://www.pivotaltracker.com/story/show/134097225)

Signed-off-by: Rob Day-Reynolds <904086f049d807ae99640dbdfde98536a5822f56@pivotal.io>

barthy1/bosh,barthy1/bosh,barthy1/bosh,barthy1/bosh