qemu-mirror : block job complete : use ready flag

qemu-mirror : block job complete : use ready flag

Since qemu 2.2, a new "ready" flag has been added to blockjob
http://git.qemu.org/?p=qemu.git;a=commit;h=ef6dbf1e46ebd1d41ab669df5bba0bbdec6bd374
to known if we can complete it.

we can't use len==offset to known if all block are mirrored, because behaviour will change soon in qemu 2.3

http://git.qemu.org/?p=qemu.git;a=commit;h=b21c76529d55bf7bb02ac736b312f5f8bf033ea2
"block/mirror: Improve progress report

Instead of taking the total length of the block device as the block
job's length, use the number of dirty sectors. The progress is now the
number of sectors mirrored to the target block device. Note that this
may result in the job's length increasing during operation, which is
however in fact desirable.
"

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>