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