Description of problem:
qga command "guest-set-user-password" can't get exepected result.  Instead, it gives a error msg:
"child process has failed to set user password"

Version-Release number of selected component (if applicable):

qemu-guest-agent-0.12.1.2-2.488.el6.x86_64 

How reproducible:
2/2

Steps to Reproduce:
1.start up guest 
/usr/libexec/qemu-kvm \
...
    -device virtio-serial-pci,id=virtio_serial_pci0,bus=pci.0,addr=03,disable-legacy=off,disable-modern=on \
    -chardev socket,path=virtio_port-org.qemu.guest_agent.0,nowait,id=id8bMDQm,server \
    -device virtserialport,bus=virtio_serial_pci0.0,name=org.qemu.guest_agent.0,chardev=id8bMDQm,id=idJV2WfH \
...

2.Send guest-set-user-password the qga
{"execute": "guest-set-user-password", "arguments": {"crypted": false, "username": "root", "password": "a3ZtYXV0b3Rlc3Q="}} 

3.Check the return value of command

Actual results:
{"execute": "guest-set-user-password", "arguments": {"crypted": false, "username": "root", "password": "a3ZtYXV0b3Rlc3Q="}}
{"error": {"class": "GenericError", "desc": "child process has failed to set user password", "data": {"message": "child process has failed to set user password"}}} 

Expected results:
It returns good ,and the password can be set

Additional info:

Guest boot cmd:
/usr/libexec/qemu-kvm \
    -machine pc  \
    -nodefaults  \
    -vga cirrus  \
    -device virtio-serial-pci,id=virtio_serial_pci0,bus=pci.0,addr=03,disable-legacy=off,disable-modern=on \
    -chardev socket,path=virtio_port-org.qemu.guest_agent.0,nowait,id=id8bMDQm,server \
    -device virtserialport,bus=virtio_serial_pci0.0,name=org.qemu.guest_agent.0,chardev=id8bMDQm,id=idJV2WfH \
    -device ich9-usb-ehci1,id=usb1,addr=1d.7,multifunction=on,bus=pci.0 \
    -device ich9-usb-uhci1,id=usb1.0,multifunction=on,masterbus=usb1.0,addr=1d.0,firstport=0,bus=pci.0 \
    -device ich9-usb-uhci2,id=usb1.1,multifunction=on,masterbus=usb1.0,addr=1d.2,firstport=2,bus=pci.0 \
    -device ich9-usb-uhci3,id=usb1.2,multifunction=on,masterbus=usb1.0,addr=1d.4,firstport=4,bus=pci.0 \
    -drive id=drive_image1,if=none,snapshot=on,aio=threads,cache=none,format=qcow2,file=/usr/share/avocado/data/avocado-vt/images/RHEL-Server-6.8-64-virtio.qcow2 \
    -device virtio-blk-pci,id=image1,drive=drive_image1,bootindex=0,bus=pci.0,addr=04,disable-legacy=off,disable-modern=on \
    -device virtio-net-pci,mac=9a:dd:de:df:e0:e1,id=idEm3Sb0,vectors=4,netdev=idHCUDn2,bus=pci.0,addr=05,disable-legacy=off,disable-modern=on \
    -netdev tap,id=idHCUDn2,vhost=on \
    -m 2048  \
    -smp 4,maxcpus=4,cores=2,threads=1,sockets=2  \
    -cpu 'SandyBridge',+kvm_pv_unhalt \
    -device usb-tablet,id=usb-tablet1,bus=usb1.0,port=1  \
    -vnc :0  \
    -rtc base=utc,clock=host,driftfix=slew  \
    -boot order=cdn,once=c,menu=off,strict=off \
    -enable-kvm