kvm tools, qcow: Add the support for copy-on-write cluster

kvm tools, qcow: Add the support for copy-on-write cluster

When meeting request to write the cluster without copied flag,
allocate a new cluster and write original data with modification
to the new cluster. This also adds support for the writing operation
of the qcow2 compressed image. After testing, image file can pass
through "qemu-img check". The performance is needed to be improved.

Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>