kvm tools: Add support for the read operation of qcow and qcow2 compressed image

This patch adds the decompression operation when confirming the qcow or
qcow2 image is compressed. This patch also divides the read cluster
fucntion into two which are respective for qcow and qcow2 in order to be
convenient to support these two kind images. Add some macros for qcow.

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