Resolved device mapping issues (Xen) in aws_ebs_raid LWRP

I've found a couple of issues while trying to use the aws_ebs_raid LWRP, especially when making multiple raid devices during the same chef-client run.

Additionally, I changed the way the ruby_block[new_resource.name] runs to sleep only if necessary (it was always sleeping 10 seconds before checking for presence of devices.