Support LibVirt connect URI parameter (fix #291)

Try to get the value of a connect key from virtualization config options
if it has been specified as a reference, and use that with virsh as
connect URI. Also tries to fall back to use the default LibVirt
connection parameters.
Merge pull request #307 from ferki/libvirt-connect

Hi Ferki,

thank you very much for your patch. I've just merged it into development branch.