adds support for libvirt tls connections

i use tls connections for libvirt[1], and wanted webvirtmgr to work in that environment:

![screen shot 2014-05-26 at 00 10 41](https://cloud.githubusercontent.com/assets/7424193/3079069/c255888c-e48b-11e3-9dbb-fb333cd0d3fb.png)
[1] http://libvirt.org/remote.html