avocado.utils.virt: Be quiet if libvirt is not present

Avoid to print message that libvirt is not installed,
just disable "run_vm plugin" and stop polluting the output.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #92 from ruda/has_virt_support

avocado.utils.virt: Be quiet if libvirt is not present