Compression in qcow2 requires image length to be a multiple of the
cluster size.  Lift this requirement by zero-padding the final cluster
when necessary.  The virtual disk size is still not cluster-aligned, so
the guest cannot access the zero sectors.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>