virt plugin: Disable tests momentarily

virt plugin: Disable tests momentarily

Old libvirts (<= 0.10.z, such as the version shipped on debian wheezy)
depend on libnl-1. There is a small memory leak caused by the
initialization of the library, and we have no means to avoid it.
This causes valgrind to complain in the tests, creating a false
positive.

Anyway, libvirt switched to libnl-3, which should also fix this leak.
The simplest solution is just to disable those tests until we
can depend on libnl-3 enabled libvirt.