This patch adds code to look up data cluster offsets in the image via
the L1/L2 tables.  The L2 tables are writethrough cached in memory for
performance (each read/write requires a lookup so it is essential to
cache the tables).

With cluster lookup code in place it is possible to implement
bdrv_is_allocated() to query the number of contiguous
allocated/unallocated clusters.
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>