Description of problem:
On current qemu-kvm-rhev version, the default io throttling group name is null, which makes all disks under io throttling but without a group name in the same group, which might be not intended.


And in docs/throttle.txt of the qemu-kvm code, there is following introduction:

Applying I/O limits to groups of disks
--------------------------------------
......
If this parameter is not specified, then the device name (i.e. 'virtio0', 'ide0-hd0') will be used as the group name.
......
 

Version-Release number of selected component (if applicable):
kernel: 3.10.0-458.el7.x86_64
qemu-kvm-rhev: qemu-kvm-rhev-2.6.0-11.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Boot a guest with 2 disks under different io throttling parameters, but without group names:

/usr/libexec/qemu-kvm -name rhel7.3 -enable-kvm -m 4G -smp 4 -uuid ae361e8d-288b-4e70-8783-bda0d450ce39 -nodefconfig --nodefaults -rtc base=utc,clock=vm -boot order=cd,menu=on -device virtio-scsi-pci,id=virtio_scsi_pci0,bus=pci.0,addr=03 -drive id=drive_image1,if=none,cache=none,snapshot=off,aio=native,format=qcow2,file=/home/kvm_autotest_root/images/RHEL-Server-7.3-64-virtio-scsi.qcow2 -device scsi-hd,id=image1,drive=drive_image1,bootindex=0 -device nec-usb-xhci,id=xhci -vnc 0.0.0.0:15 -monitor stdio -netdev tap,script=/etc/qemu-ifup,downscript=/etc/qemu-ifdown,id=hostnet0 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=00:52:5a:30:2e:62,bus=pci.0,addr=0x7 -vga std -device usb-tablet,id=tablet0 -object rng-random,id=rng0,filename=/dev/random -device virtio-rng-pci,rng=rng0,id=rng0 -drive id=drive_image2,if=none,cache=none,snapshot=off,aio=native,format=raw,file=/home/image1,bps=10240,bps_max=102400,iops=100,throttling.bps-total-max-length=2 -device virtio-blk-pci,id=image2,drive=drive_image2,bus=pci.0,addr=10 -drive id=drive_image3,if=none,cache=none,snapshot=off,aio=native,format=raw,file=/home/image2,bps=2048,bps_max=204800,iops=200,throttling.bps-total-max-length=4 -device virtio-blk-pci,id=image3,drive=drive_image3,bus=pci.0,addr=09 -chardev socket,id=qmpmonitor1,path=/tmp/monitor11,server,nowait -mon chardev=qmpmonitor1,mode=control

2. Check blocks info in hmp and qmp:
# (qemu) info block
drive_image1 (#block199): /home/kvm_autotest_root/images/RHEL-Server-7.3-64-virtio-scsi.qcow2 (qcow2)
    Cache mode:       writeback, direct

drive_image2 (#block397): /home/image1 (raw)
    Cache mode:       writethrough, direct
    I/O throttling:   bps=2048 bps_rd=0 bps_wr=0 bps_max=204800 bps_rd_max=0 bps_wr_max=0 iops=200 iops_rd=0 iops_wr=0 iops_max=20 iops_rd_max=0 iops_wr_max=0 iops_size=0 group=

drive_image3 (#block545): /home/image2 (raw)
    Cache mode:       writethrough, direct
    I/O throttling:   bps=2048 bps_rd=0 bps_wr=0 bps_max=204800 bps_rd_max=0 bps_wr_max=0 iops=200 iops_rd=0 iops_wr=0 iops_max=20 iops_rd_max=0 iops_wr_max=0 iops_size=0 group=

# nc -U /tmp/monitor11
{"QMP": {"version": {"qemu": {"micro": 0, "minor": 6, "major": 2}, "package": " (qemu-kvm-rhev-2.6.0-11.el7)"}, "capabilities": []}}
{"execute":"qmp_capabilities"}
{"return": {}}
{"execute":"query-block"}
{"return": [{"io-status": "ok", "device": "drive_image1", "locked": false, "removable": false, "inserted": {"iops_rd": 0, "detect_zeroes": "off", "image": {"virtual-size": 21474836480, "filename": "/home/kvm_autotest_root/images/RHEL-Server-7.3-64-virtio-scsi.qcow2", "cluster-size": 65536, "format": "qcow2", "actual-size": 5543170048, "format-specific": {"type": "qcow2", "data": {"compat": "1.1", "lazy-refcounts": false, "refcount-bits": 16, "corrupt": false}}, "dirty-flag": false}, "iops_wr": 0, "ro": false, "node-name": "#block199", "backing_file_depth": 0, "drv": "qcow2", "iops": 0, "bps_wr": 0, "write_threshold": 0, "encrypted": false, "bps": 0, "bps_rd": 0, "cache": {"no-flush": false, "direct": true, "writeback": true}, "file": "/home/kvm_autotest_root/images/RHEL-Server-7.3-64-virtio-scsi.qcow2", "encryption_key_missing": false}, "type": "unknown"}, {"io-status": "ok", "device": "drive_image2", "locked": false, "removable": false, "inserted": {"iops_rd": 0, "detect_zeroes": "off", "iops_max_length": 1, "bps_max_length": 4, "image": {"virtual-size": 21474836480, "filename": "/home/image1", "format": "raw", "actual-size": 0, "dirty-flag": false}, "iops_wr": 0, "group": "", "ro": false, "node-name": "#block397", "backing_file_depth": 0, "drv": "raw", "bps_max": 204800, "iops": 200, "bps_wr": 0, "write_threshold": 0, "encrypted": false, "bps": 2048, "bps_rd": 0, "cache": {"no-flush": false, "direct": true, "writeback": true}, "iops_max": 20, "file": "/home/image1", "encryption_key_missing": false}, "type": "unknown"}, {"io-status": "ok", "device": "drive_image3", "locked": false, "removable": false, "inserted": {"iops_rd": 0, "detect_zeroes": "off", "iops_max_length": 1, "bps_max_length": 4, "image": {"virtual-size": 21474836480, "filename": "/home/image2", "format": "raw", "actual-size": 0, "dirty-flag": false}, "iops_wr": 0, "group": "", "ro": false, "node-name": "#block545", "backing_file_depth": 0, "drv": "raw", "bps_max": 204800, "iops": 200, "bps_wr": 0, "write_threshold": 0, "encrypted": false, "bps": 2048, "bps_rd": 0, "cache": {"no-flush": false, "direct": true, "writeback": true}, "iops_max": 20, "file": "/home/image2", "encryption_key_missing": false}, "type": "unknown"}]}


Actual results:
In step2, the group names of both disks are null, and both disks use the same io throttling parameters, i.e. drive_image2 use the ones of drive_image3; so both disks are treated in the same null group.

Expected results:
drive_image2 and drive_image3 should be in different groups

Additional info: