Allow the default (kvm) for nova_virt_type to be used

* Changes the 2 kvm tasks to use "virt_type" instead of "nova_virt_type"
* This var is specified from group_vars file and is defaulted.

Closes-Bug: #242
Merge pull request #243 from andymcc/nova_virt_type

Allow the default (kvm) for nova_virt_type to be used