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Just like qemu-img convert uses bdrv_has_zero_init() in order to check whether
it can assume that not explicitly written clusters are zeroed, mirroring needs
to do the same. Otherwise mirroring to a raw block device with uninitialised
data doesn't yield the correct result.

Manual test case:
$ qemu-img create -f qcow2 /tmp/test.qcow2 64M
$ dd if=/dev/urandom of=/tmp/test.raw bs=1M count=64
$ sudo losetup /dev/loop0 /tmp/test.raw
$ sudo chmod 666 /dev/loop0
$ x86_64-softmmu/qemu-system-x86_64 -hda /tmp/test.qcow2 -monitor stdio
(qemu) drive_mirror ide0-hd0 /dev/loop0 raw
Formatting '/dev/loop0', fmt=raw size=67108864 
(qemu) block_job_complete ide0-hd0
(qemu) quit
$ sudo hexdump -n 512 /dev/loop0

Expected result:
Only zero bytes are on the block device

Actual result:
The random bytes are still there

Can reproduce by using qemu-kvm-1.5.0-2.

Before do mirror
#hexdump -n 512 /root/zhangjunyi/test.raw 
0000000 2847 67ae c62e b392 554f e6d8 95aa e80a
0000010 a68e c5f3 33d0 b844 6464 a216 7625 1640
0000020 20f2 1045 66e9 cf58 74c6 038d f0c1 cd31
..............................

After mirror
#hexdump -n 512 /root/zhangjunyi/test.raw 
0000000 0000 0000 0000 0000 0000 0000 0000 0000
*
0000200