kvm tools, qcow: Rename struct qcow_l2_cache to struct qcow_l2_table

kvm tools, qcow: Rename struct qcow_l2_cache to struct qcow_l2_table

This patch renames 'struct qcow_l2_cache' to 'struct qcow_l2_table' in
preparation for replacing the untyped L2 table arrays with the struct.

Cc: Asias He <asias.hejun@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Prasad Joshi <prasadjoshi124@gmail.com>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>