Description of problem:
Currently there is no alias for burst_length used in qemu-kvm command line, while there are ones for the burst, such as bps_max, iops_max etc; however, there are similar ones in qmp for the burst_length already, i.e. bps_max_length, iops_max_length etc. So suggest to add the alias in qemu-kvm command. 

Version-Release number of selected component (if applicable):
kernel: 3.10.0-406.el7.x86_64
qemu-kvm-rhev: qemu-kvm-rhev-2.6.0-4.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Start a guest with burst length:

# /usr/libexec/qemu-kvm -name rhel7.3 -enable-kvm -m 4G -smp 4 -uuid ae361e8d-288b-4e70-8783-bda0d450ce38 -nodefconfig --nodefaults -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/rhel7.3 -device scsi-hd,id=image1,drive=drive_image1,bootindex=0 -drive file=/home/kvm_autotest_root/iso/linux/RHEL-7.2-20151030.0-Server-x86_64-dvd1.iso,media=cdrom,id=cdrom,if=none -device ide-drive,drive=cdrom,bootindex=1 -vnc 0.0.0.0:14 -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:61,bus=pci.0,addr=0x7 -drive id=drive_image2,if=none,cache=none,snapshot=off,aio=native,format=qcow2,file=/home/disk1,**bps=102400,bps_max=512000,throttling.bps-total-max-length=10** -device scsi-hd,id=image2,drive=drive_image2 -chardev socket,id=qmpmonitor1,path=/tmp/monitor1,server,nowait -mon chardev=qmpmonitor1,mode=control -vga cirrus

2. After the guest boots up, check iothrottling status in qmp:

# nc -U /tmp/monitor1
{"QMP": {"version": {"qemu": {"micro": 0, "minor": 6, "major": 2}, "package": " (qemu-kvm-rhev-2.6.0-4.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/rhel7.3", "cluster-size": 65536, "format": "qcow2", "actual-size": 1461977088, "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": "#block147", "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/rhel7.3", "encryption_key_missing": false}, "type": "unknown"}, {"io-status": "ok", "device": "cdrom", "locked": true, "removable": true, "inserted": {"iops_rd": 0, "detect_zeroes": "off", "image": {"virtual-size": 4043309056, "filename": "/home/kvm_autotest_root/iso/linux/RHEL-7.2-20151030.0-Server-x86_64-dvd1.iso", "format": "raw", "actual-size": 4051255296, "dirty-flag": false}, "iops_wr": 0, "ro": true, "node-name": "#block362", "backing_file_depth": 0, "drv": "raw", "iops": 0, "bps_wr": 0, "write_threshold": 0, "encrypted": false, "bps": 0, "bps_rd": 0, "cache": {"no-flush": false, "direct": false, "writeback": true}, "file": "/home/kvm_autotest_root/iso/linux/RHEL-7.2-20151030.0-Server-x86_64-dvd1.iso", "encryption_key_missing": false}, "tray_open": false, "type": "unknown"}, {"io-status": "ok", "device": "drive_image2", "locked": false, "removable": false, "inserted": {"iops_rd": 0, "detect_zeroes": "off", **"bps_max_length": 10**, "image": {"virtual-size": 1073741824, "filename": "/home/disk1", "cluster-size": 65536, "format": "qcow2", "actual-size": 118824960, "format-specific": {"type": "qcow2", "data": {"compat": "1.1", "lazy-refcounts": false, "refcount-bits": 16, "corrupt": false}}, "dirty-flag": false}, "iops_wr": 0, "group": "", "ro": false, "node-name": "#block528", "backing_file_depth": 0, "drv": "qcow2", "bps_max": 512000, "iops": 0, "bps_wr": 0, "write_threshold": 0, "encrypted": false, "bps": 102400, "bps_rd": 0, "cache": {"no-flush": false, "direct": true, "writeback": true}, "file": "/home/disk1", "encryption_key_missing": false}, "type": "unknown"}]}

3. Quit the guest and try to restart it with burst_length in the style as that in qmp, i.e. **bps=102400,bps_max=512000,bps_max_length=10** 


Actual results:
In step3, it's a failure to start the guest

Expected results:


Additional info: