Description of problem:
[SR-IOV] - VF that was attached to VM via HostDevices sub tab considered as free and can cause sometimes to other VM to fail running if it trying to run with a passthrough vNIC, because it tries to use the directly attached VF.

Version-Release number of selected component (if applicable):
3.6.2

How reproducible:
Sometimes 

Steps to Reproduce:
1. Enable few VFs on capable sr-iov host 
2. Attach 1 VF directly to a VM via HostDevices sub tab
3. Run VM
4. Add vNIC with passthrough profile to other VM and try to run it 

Actual results:
Sometimes VM failing to run, because the directly attached VF considered as free

Expected results:
VF that was attached directly to VM via HostDevices sub tab should be considered 
as non-free