qcow2: Avoid making the L1 table too big

We refuse to open images whose L1 table we deem "too big". Consequently,
we should not produce such images ourselves.

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20160615153630.2116-3-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
[mreitz: Added QEMU_BUILD_BUG_ON()]
Signed-off-by: Max Reitz <mreitz@redhat.com>

(cherry picked from commit 84c26520)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>