qcow2: Validate refcount table offset

qcow2: Validate refcount table offset

The end of the refcount table must not exceed INT64_MAX so that integer
overflows are avoided.

Also check for misaligned refcount table. Such images are invalid and
probably the result of data corruption. Error out to avoid further
corruption.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 8c7de28305a514d7f879fdfc677ca11fbf60d2e9)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>