kvm tools, qcow: Move L2 cache to 'struct qcow_l1_table'

In preparation for refcount block cache, move L2 cache data structures to
'struct qcow_l1_table'.

Cc: Prasad Joshi <prasadjoshi124@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>