qcow2: Add plausibility check for L1/L2 entries (Kevin Wolf)

From: Kevin Wolf <kwolf@redhat.com>

All L1 and L2 entries must point at the start of a cluster. If there is some
offset into the cluster, the entry is corrupted.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7217 c046a42c-6fe2-441c-8c8c-71466251a162
