storage_file: add support to probe cluster_size from QCOW2 images

storage_file: add support to probe cluster_size from QCOW2 images

From QEMU docs/interop/qcow2.txt :

   Byte  20 - 23:   cluster_bits
                    Number of bits that are used for addressing an offset
                    within a cluster (1 << cluster_bits is the cluster size).

With this patch libvirt will be able to report the current cluster_size
for all existing storage volumes managed by storage driver.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>