CLOUDSTACK-1340: Compress QCOW2 for KVM

As suggested by Marcus, by compression qcow2, we end up having smaller disk image.
I'm still archiving the qcow2 as bz2 because our installation manual still ask
users to cloud-install-sys-tmplt which expects a *.qcow2.bz2

Signed-off-by: Rohit Yadav <bhaisaab@apache.org>