Description of problem:
Hit two issues mirroring when the source have no enough space
1. The value of "offset" reduced untill the I/O error occurred
2. The base file didn't change when mirror completed 


Version-Release number of selected component (if applicable):
kernel-2.6.32-620.el6.x86_64
qemu-kvm-rhev-0.12.1.2-2.489.el6.x86_64

How reproducible:
1. The value of "offset" reduced untill the I/O error occurred (3/6)
2. The base file didn't change when mirror complete  (2/6)

Steps to Reproduce:

Pre: 
1.The path which contain the image is limited, such as 12G
2.The image 10G

1. Boot vm with following cli:
/usr/libexec/qemu-kvm \
-name rhel6.6.0 \
-machine rhel6.6.0,accel=kvm \
-realtime mlock=off \
-cpu SandyBridge \
-m 4G   \
-smp 4,cores=1,threads=1,sockets=4  \
-uuid 49a3438a-70a3-4ba8-92ce-3a05e0934608 \
-nodefaults \
-rtc base=utc,driftfix=slew \
-monitor unix:/tmp/monitor-unix,server,nowait \
-serial unix:/tmp/serial0,server,nowait \
-global kvm-pit.lost_tick_policy=discard \
-global PIIX4_PM.disable_s3=1 \
-global PIIX4_PM.disable_s4=1 \
-boot order=c,menu=on,strict=on \
-device virtio-scsi-pci,id=scsi \
-drive file=/mnt/rhel68bk.qcow2,if=none,format=qcow2,id=drive-virtio-scsi,werror=stop,rerror=stop \
-device scsi-hd,drive=drive-virtio-scsi,id=virtio-scsi,bus=scsi.0,scsi-id=0 \
-netdev tap,id=hostnet0,vhost=on \
-device virtio-net-pci,netdev=hostnet0,id=net0,vectors=0,mac=00:1a:4a:42:0a:00 \
-monitor stdio \
-qmp tcp:0:4466,server,nowait -serial unix:/tmp/ttym,server,nowait \
-vga qxl \
-spice port=5910,password=redhat-vga,disable-ticketing \
-k en-us \


2. In guest, dd a large file, make sure large file+ image file > the path space
such as "dd if=/dev/urandom of=/home/test.raw bs=3G"

3. Im qmp, mirror the base to a path which have enough space

{ "execute" : "__com.redhat_drive-mirror", "arguments" : { "device" : "drive-virtio-scsi","target" : "/home/0223/target/sn1", "format": "qcow2"} }

4. query the mirror job in qmp
{ "execute" : "query-block-jobs", "arguments" : {} }

5. In qmp:
{ "execute": "__com.redhat_drive-reopen", "arguments": {"device": "drive-virtio-scsi", "new-image-file": "/home/0223/target/sn1" } }
and check the disk in hmp "info block"

Actual results:
1. After step4, the　"offset" value reduced until the I/O error occurred and the "offset" value increased after the I/O error occurred.

{ "execute" : "query-block-jobs", "arguments" : {} }
{"return": [{"device": "drive-virtio-scsi", "len": 21474836480, "offset": 13071941632, "speed": 0, "type": "mirror"}]}
{ "execute" : "query-block-jobs", "arguments" : {} }
{"return": [{"device": "drive-virtio-scsi", "len": 21474836480, "offset": 13071941632, "speed": 0, "type": "mirror"}]}
{ "execute" : "query-block-jobs", "arguments" : {} }
{"return": [{"device": "drive-virtio-scsi", "len": 21474836480, "offset": 13071941632, "speed": 0, "type": "mirror"}]}
{ "execute" : "query-block-jobs", "arguments" : {} }
{"return": [{"device": "drive-virtio-scsi", "len": 21474836480, "offset": 12816023552, "speed": 0, "type": "mirror"}]}
{ "execute" : "query-block-jobs", "arguments" : {} }
{"return": [{"device": "drive-virtio-scsi", "len": 21474836480, "offset": 12801015808, "speed": 0, "type": "mirror"}]}
{ "execute" : "query-block-jobs", "arguments" : {} }
{"return": [{"device": "drive-virtio-scsi", "len": 21474836480, "offset": 12801015808, "speed": 0, "type": "mirror"}]}
{ "execute" : "query-block-jobs", "arguments" : {} }
{"return": [{"device": "drive-virtio-scsi", "len": 21474836480, "offset": 12774146048, "speed": 0, "type": "mirror"}]}
{ "execute" : "query-block-jobs", "arguments" : {} }
{"return": [{"device": "drive-virtio-scsi", "len": 21474836480, "offset": 12774146048, "speed": 0, "type": "mirror"}]}
{ "execute" : "query-block-jobs", "arguments" : {} }
{"return": [{"device": "drive-virtio-scsi", "len": 21474836480, "offset": 12660047872, "speed": 0, "type": "mirror"}]}
{"timestamp": {"seconds": 1457072345, "microseconds": 365927}, "event": "BLOCK_IO_ERROR", "data": {"device": "drive-virtio-scsi", "__com.redhat_debug_info": {"message": "No space left on device", "errno": 28}, "__com.redhat_reason": "enospc", "operation": "write", "action": "stop"}}
{ "execute" : "query-block-jobs", "arguments" : {} }
{"return": [{"device": "drive-virtio-scsi", "len": 21474836480, "offset": 19245105152, "speed": 0, "type": "mirror"}]}
{ "execute" : "query-block-jobs", "arguments" : {} }
{"return": [{"device": "drive-virtio-scsi", "len": 21474836480, "offset": 19266928640, "speed": 0, "type": "mirror"}]}
{ "execute" : "query-block-jobs", "arguments" : {} }
{"return": [{"device": "drive-virtio-scsi", "len": 21474836480, "offset": 20651966464, "speed": 0, "type": "mirror"}]}

2. After the "len" value equal to the "offset" value

{ "execute": "__com.redhat_drive-reopen", "arguments": {"device": "drive-virtio-scsi", "new-image-file": "/home/0223/target/sn1" } }
and check the disk in hmp "info block"
drive-virtio-scsi: removable=0 io-status=nospace file=/mnt/rhel68bk.qcow2 ro=0 drv=qcow2 encrypted=0 bps=0 bps_rd=0 bps_wr=0 iops=0 iops_rd=0 iops_wr=0
(qemu) info bl
block       blockstats  block-jobs  
(qemu) info block-jobs 
No active jobs
(qemu) info block
drive-virtio-scsi: removable=0 io-status=nospace file=/mnt/rhel68bk.qcow2 ro=0 drv=qcow2 encrypted=0 bps=0 bps_rd=0 bps_wr=0 iops=0 iops_rd=0 iops_wr=0
(qemu) c
(qemu) block I/O error in device 'drive-virtio-scsi': No space left on device (28)
block I/O error in device 'drive-virtio-scsi': No space left on device (28)
block I/O error in device 'drive-virtio-scsi': No space left on device (28)
block I/O error in device 'drive-virtio-scsi': No space left on device (28)



Expected results:
1. After step4, the "offset" value increased 
2. After step5. the disk file can change to the target file and No I/O error

Additional info: