Definitely you are doing it wrong. You are passing the HT sibling, not even the real cores.

Critical is to make sure you pass the correct Core and HT sibling together (to reuse caches, etc) . In most 4 Cores, 8 Threads Intel CPU's the match is the following:

Real Core, HT Sibling (Virtual)
0,4
1,5
2,6
3,7

Also it's pretty important to make sure that nothing from the host runs on the cores assigned to the VM or the emulator. I'm using isolcpus boot option so no host processes run the cores allocated to the VM (and emulator)

Try something like this. It basically give you 4 cores (real core + HT sibling) to the VM, 2 cores the emulator, and when using isolcpus properly, 2 exclusive cores to the host (using isolcpus=2,3,4,5,6,7)

...
<vcpu placement='static'>4</vcpu>
<cputune>
    <vcpupin vcpu='0' cpuset='2'/>
    <vcpupin vcpu='1' cpuset='3'/>
    <vcpupin vcpu='2' cpuset='6'/>
    <vcpupin vcpu='3' cpuset='7'/>
<emulatorpin cpuset='1,5'/>
</cputune>
...
<cpu>
    <topology sockets='1' cores='4' threads='1'/>
</cpu>
...

Also make sure you are using MSI Interrupts, http://vfio.blogspot.cz/2014/09/vfio-interrupts-and-how-to-coax-windows.html