qcow2: record fragmentation statistics during check

The qemu-img check command can display fragmentation statistics:
 * Total number of clusters in virtual disk
 * Number of allocated clusters
 * Number of fragmented clusters

This patch adds fragmentation statistics support to qcow2.

Compressed and normal clusters count as allocated.  Zero clusters are
not counted as allocated unless their L2 entry has a non-zero offset
(e.g. preallocation).

Only the current L1 table counts towards the statistics - snapshots are
ignored.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>