Description of problem:
Boot guest with -cpu host in host which nested enabled, when run the libguestfs-test-tool, the tool hangs at 
...
[    1.347651] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
...
Version-Release number of selected component (if applicable):
kernel of both host and guest:
# uname -r
3.10.0-118.el7.x86_64

# rpm -q qemu-kvm-rhev
qemu-kvm-rhev-1.5.3-60.el7ev.x86_64

How reproducible:
100%

Steps to Reproduce:
1.Enable nested in host:
# modprobe -r kvm_intel
# modprobe kvm_intel nested=1
# cat /sys/module/kvm_intel/parameters/nested 
Y

2.Boot rhel7 guest with -cpu host:
# /usr/libexec/qemu-kvm -S -name rhel7 -M pc -nodefaults -vga std  -device ich9-usb-uhci1,id=usb1,bus=pci.0,addr=03 -drive id=drive_image1,if=none,cache=none,aio=native,file=/home/rhel70326cp1.qcow2_v3 -device virtio-blk-pci,id=image1,drive=drive_image1,bus=pci.0,addr=04 -device virtio-net-pci,mac=16:33:3f:09:12:78,id=vnet0,netdev=hostdev0,bus=pci.0,addr=05 -netdev tap,id=hostdev0,vhost=on,script=/etc/qemu-ifup -m 4G -smp 4,sockets=1,cores=4,threads=1 -cpu host  -device usb-tablet,id=usb-tablet1,bus=usb1.0,port=1 -vnc :3 -rtc base=localtime,clock=host,driftfix=slew -boot menu=on -enable-kvm -monitor stdio -qmp unix:/tmp/q1,server,nowait -device virtio-balloon-pci,id=b1

3.Inside guest:
# libguestfs-test-tool

Actual results:
The tool hangs at 

...
[    1.347651] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
...

Expected results:
libguestfs-test-tool can finish successfully

Additional info:
Host infos (24 cpus in total)
# cat /proc/cpuinfo  
processor	: 0
vendor_id	: GenuineIntel
cpu family	: 6
model		: 45
model name	: Intel(R) Xeon(R) CPU E5-2620 0 @ 2.00GHz
stepping	: 7
microcode	: 0x710
cpu MHz		: 2221.093
cache size	: 15360 KB
physical id	: 0
siblings	: 12
core id		: 0
cpu cores	: 6
apicid		: 0
initial apicid	: 0
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 dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc aperfmperf eagerfpu pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dtherm tpr_shadow vnmi flexpriority ept vpid
bogomips	: 4000.13
clflush size	: 64
cache_alignment	: 64
address sizes	: 46 bits physical, 48 bits virtual
power management:

.....
.....

processor	: 23
vendor_id	: GenuineIntel
cpu family	: 6
model		: 45
model name	: Intel(R) Xeon(R) CPU E5-2620 0 @ 2.00GHz
stepping	: 7
microcode	: 0x710
cpu MHz		: 1460.156
cache size	: 15360 KB
physical id	: 1
siblings	: 12
core id		: 5
cpu cores	: 6
apicid		: 43
initial apicid	: 43
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 dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc aperfmperf eagerfpu pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dtherm tpr_shadow vnmi flexpriority ept vpid
bogomips	: 4004.59
clflush size	: 64
cache_alignment	: 64
address sizes	: 46 bits physical, 48 bits virtual
power management:


2.Test with -cpu SandyBridge/Penryn, won't hit this issue.

3.Will attach the whole logs of -cpu host and -cpu SandyBridge