Properly set default virtualization type to hvm in create_stemcell
Properly set default virtualization type to hvm in create_stemcell

[#116390209](https://www.pivotaltracker.com/story/show/116390209)

Signed-off-by: Edwin Xie <6bc24e9c160281972837d9e13287696d26612564@pivotal.io>

cloudfoundry-incubator/bosh-aws-cpi-release,cloudfoundry-incubator/bosh-aws-cpi-release,cloudfoundry-incubator/bosh-aws-cpi-release