Description of problem:

i met this when i run autotest script "system_powerdown", this case will shutdown guest and wait for 360 seconds.
using hmp command "system_powerdown" and can't shutdown the guest

Version-Release number of selected component (if applicable):
qemu-kvm-rhev-2.3.0-31.el7.x86_64
kernel-3.10.0-327.el7.x86_64

How reproducible:

i run 10 times and i met one time.

Steps to Reproduce:
1.1.boot up the guest with commandline:

/usr/libexec/qemu-kvm \
    -S  \
    -name 'virt-tests-vm1'  \
    -sandbox off  \
    -machine pc  \
    -nodefaults  \
    -vga qxl  \
    -chardev socket,id=qmp_id_qmpmonitor1,path=/tmp/monitor-qmpmonitor1-20151103-155855-5Qec1sex,server,nowait \
    -mon chardev=qmp_id_qmpmonitor1,mode=control  \
    -chardev socket,id=qmp_id_catch_monitor,path=/tmp/monitor-catch_monitor-20151103-155855-5Qec1sex,server,nowait \
    -mon chardev=qmp_id_catch_monitor,mode=control \
    -device pvpanic,ioport=0x505,id=idrUyyM1  \
    -chardev socket,id=serial_id_serial0,path=/tmp/serial-serial0-20151103-155855-5Qec1sex,server,nowait \
    -device isa-serial,chardev=serial_id_serial0  \
    -chardev socket,id=seabioslog_id_20151103-155855-5Qec1sex,path=/tmp/seabios-20151103-155855-5Qec1sex,server,nowait \
    -device isa-debugcon,chardev=seabioslog_id_20151103-155855-5Qec1sex,iobase=0x402 \
    -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,cache=none,snapshot=off,aio=native,format=qcow2,file=/home/autotest/client/tests/virt/shared/data/images/win2012-64r2-virtio.qcow2 \
    -device virtio-blk-pci,id=image1,drive=drive_image1,bootindex=0,bus=pci.0,addr=03,disable-legacy=off,disable-modern=on \
    -device virtio-net-pci,mac=9a:bf:c0:c1:c2:c3,id=idIor1id,vectors=4,netdev=id9LR8kS,bus=pci.0,addr=04,disable-legacy=off,disable-modern=on  \
    -netdev tap,id=id9LR8kS,vhost=on,vhostfd=24,fd=23  \
    -m 32768  \
    -smp 16,maxcpus=16,cores=8,threads=1,sockets=2  \
    -cpu 'Opteron_G4',+kvm_pv_unhalt,hv_spinlocks=0x1fff,hv_vapic,hv_time \
    -drive id=drive_cd1,if=none,snapshot=off,aio=native,media=cdrom,file=/home/autotest/client/tests/virt/shared/data/isos/windows/winutils.iso \
    -device ide-cd,id=cd1,drive=drive_cd1,bootindex=1,bus=ide.0,unit=0 \
    -device usb-tablet,id=usb-tablet1,bus=usb1.0,port=1  \
    -spice port=3000,password=123456,addr=0,image-compression=auto_glz,zlib-glz-wan-compression=auto,streaming-video=all,agent-mouse=on,playback-compression=on,ipv4  \
    -rtc base=localtime,clock=host,driftfix=slew  \
    -boot order=cdn,once=c,menu=off,strict=off \
    -enable-kvm
2.in hmp monitor, send "system_powerdown"


Actual results:
guest stays on "shuting down" screen when i check it using remote-viewer

Expected results:

guest could shutdown successfully.

Additional info:
hostinfo:

processor	: 31
vendor_id	: AuthenticAMD
cpu family	: 21
model		: 1
model name	: AMD Opteron(TM) Processor 6272                 
stepping	: 2
microcode	: 0x6000626
cpu MHz		: 2100.033
cache size	: 2048 KB
physical id	: 1
siblings	: 16
core id		: 7
cpu cores	: 8
apicid		: 79
initial apicid	: 47
fpu		: yes
fpu_exception	: yes
cpuid level	: 13
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm constant_tsc rep_good nopl nonstop_tsc extd_apicid amd_dcm aperfmperf pni pclmulqdq monitor ssse3 cx16 sse4_1 sse4_2 popcnt aes xsave avx lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs xop skinit wdt lwp fma4 nodeid_msr topoext perfctr_core perfctr_nb arat cpb hw_pstate npt lbrv svm_lock nrip_save tsc_scale vmcb_clean flushbyasid decodeassists pausefilter pfthreshold