Start up of nested VM fails oftedn in case of Centos7 tests:

- affects VPP vhostuser tests

    - not VIRL server load related, as happens even under moderate load;

- does not happen on nested VM-Ubuntu16.04;

- likely due to Centos7 Qemu version

    - qemu-kvm.x86_64 10:1.5.3-126.el7_3.6

     - Ubuntu qemu version

        -  qemu-kvm 1:2.5+dfsg-5ubuntu10.14

- possibly applicable Qemu bug references

        - [https://bugs.centos.org/view.php?id=12671]

        - [https://bugzilla.redhat.com/show_bug.cgi?id=1360137]

- requires more t/shooting to find the root cause and fix

We would need to prepare new Centos7 VM image with updated qemu version (2.5 if possible) that we can test for qemu start-up stability.