libvirt-machine: Connect using FD if local

If its a local libvirt machine, connect to SPICE display using file
descriptor. While the issue of spice connection being exposed to other
local users is not resolved by this patch[1], it prepares Boxes for the
real solution.

[1] Currently libvirt+Qemu mandate specifying a TCP port for the SPICE
display.

https://bugzilla.gnome.org/show_bug.cgi?id=738573