kvm tools, qcow: Delayed L2 table writeout

kvm tools, qcow: Delayed L2 table writeout

This patch delays writeout for new L2 tables like we do for L1 tables. If a L2
table has non-allocated clusters, we mark that in the in-memory L2 table but
don't actually write it to disk until the L2 table is thrown out of LRU cache
or when qcow_disk_flush() is called. That makes writes to new clusters volatile
before VIRTIO_BLK_T_FLUSH is issued without corrupting the QCOW image on I/O
error.

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>