Disable COW images for Xen

COW Images are based on QCOW2, which is not supported
by Xen (at least SUSE's version of it).
Merge pull request #264 from dirkmueller/release/roxy/master

Got +1 and was tested => merging.
Create Home directory if it is missing for Nova
get back create_user_and_dirs and fix user resource
transfer the user modification block from config.rb to compute.rb