Description of problem:
When I run VMs through virt-manager/libvirt my host system experiences disruptive disk I/O that is not related to guest disk I/O.  I'm measuring this I/O after issuing a "shutdown -H now" on the guest OS to unmount all drives, deactivate swap, and halt the kernel while leaving the VM "on".

What I get is around 180 kB/s of writes tied to the qemu-kvm process (i.e. it stops if I kill or suspend the process), even though the guest VM is clearly doing nothing.  While the total amount of writes isn't all that high, it is having an unusually-high impact on my host system's performance and I can't run more than two VMs without being significantly impaired.  If I add a network interface to the VM the disk I/O on the host will go up to around 400 kB/s of writes with a very significant impact to my host.

The command-line issued by virt-manager is:
/usr/bin/qemu-system-x86_64 --enable-kvm -S -M pc-1.0 -cpu core2duo,+lahf_lm,+rdtscp,+avx,+osxsave,+xsave,+aes,+tsc-deadline,+popcnt,+x2apic,+sse4.2,+sse4.1,+pdcm,+xtpr,+cx16,+tm2,+est,+smx,+vmx,+ds_cpl,+dtes64,+pclmuldq,+pbe,+tm,+ht,+ss,+acpi,+ds -enable-kvm -m 1024 -smp 1,sockets=1,cores=1,threads=1 -name test -uuid 6b692af0-6e77-6a38-366a-e135a50e4719 -nodefconfig -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/test.monitor,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown -device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 -drive file=/var/lib/libvirt/images/test.img,if=none,id=drive-virtio-disk0,format=raw,cache=none,aio=threads -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -vnc 127.0.0.1:0 -vga cirrus -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6

I can run an almost identical command by hand (the only change is removing "-S" so I don't have to issue a start command to the VM) and I don't get any unexpected disk I/O.  This leads me to believe that the disk I/O is somehow caused by libvirt.

system information:
CPU model: Intel(R) Core(TM) i7-2640M CPU @ 2.80GHz, four cores
RAM: 16GB
Disk: SATA, 750GB

qemu-kvm version: qemu-kvm 1.0.1
libvirt version: 0.9.11.3

host kernel version/arch: Gentoo kernel 3.3.8-gentoo, x86_64

guest: Ubuntu 12.04 LTS (GNU/Linux 3.2.0-23-generic x86_64)

/var/log/libvirt/qemu/test.log:
2012-07-20 14:25:11.631+0000: starting up
LC_ALL=C PATH=/bin:/sbin:/bin:/sbin:/usr/bin:/usr/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin:/opt/bin:/usr/x86_64-pc-linux-gnu/gcc-bin/4.5.3 HOME=/root USER=root QEMU_AUDIO_DRV=none /usr/bin/qemu-kvm -S -M pc-1.0 -cpu core2duo,+lahf_lm,+rdtscp,+avx,+osxsave,+xsave,+aes,+tsc-deadline,+popcnt,+x2apic,+sse4.2,+sse4.1,+pdcm,+xtpr,+cx16,+tm2,+est,+smx,+vmx,+ds_cpl,+dtes64,+pclmuldq,+pbe,+tm,+ht,+ss,+acpi,+ds -enable-kvm -m 1024 -smp 1,sockets=1,cores=1,threads=1 -name test -uuid 6b692af0-6e77-6a38-366a-e135a50e4719 -nodefconfig -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/test.monitor,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown -device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 -drive file=/var/lib/libvirt/images/test.img,if=none,id=drive-virtio-disk0,format=raw,cache=none,aio=threads -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -vnc 127.0.0.1:0 -vga cirrus -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6
Domain id=21 is tainted: high-privileges
char device redirected to /dev/pts/6
CPU feature tsc-deadline not found
qemu: terminating on signal 15 from pid 11853
2012-07-20 14:27:29.199+0000: shutting down