kvm tools, qcow: Delayed L1 table writeout

kvm tools, qcow: Delayed L1 table writeout

This patch moves L1 table writeout to qcow_disk_flush(). The rationale here is
that while writes to clusters that don't have L2 table allocated on-disk are
volatile until VIRTIO_BLK_T_FLUSH is issued, we never corrupt the disk image.

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>