qcow2_alloc_compressed_cluster_offset() used to return the cluster
offset for success and 0 for error. This doesn't only conflict with 0 as
a valid host offset, but also loses the error code.

Similar to the change made to qcow2_alloc_cluster_offset() for
uncompressed clusters in commit 148da7ea, make the function return
0/-errno and return the allocated cluster offset in a by-reference
parameter.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>