The fix: raise the victory flag
QEMU 4.0 added BDRV_REQ_NO_FALLBACK
to detect if
zeroing is fast, and automatically utilizes it during 'qemu-img
convert':
- No support for flag: avoid pre-zeroing
- Flag supported and succeeds (for O(1) implementation):
pre-zeroing is a win
- Flag supported but fails (for O(n) implementation): no penalty
from pre-zeroing