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.
Always load nbd module on KVM compute nodes

nbd is very useful to speed up live-migration without shared
storage, so ensure that libvirt can make use of it
Merge pull request #270 from dirkmueller/release/roxy/master

Got +1 and got smoketested => merging.