Fix for Marvin utils.py:checkVolumeSize Fix for test_01_create_volume to use the correct volume name for XenServer

Signed-off-by: SrikanteswaraRao Talluri <talluri@apache.org>
CLOUDSTACK-6671: Fixing NPE when a mapping is missing
Adding missing KVM mappings

Testing Done:
Local testing with removing CentOS mapping and launch a VM.

Signed-off-by: Nitin Mehta <nitin.mehta@citrix.com>