Add tests for virt-xxx command in libvirt.

It is related heavily with libguestfs test. It build a command and config a storage and network, then pass a cmd to lgf_command(). Before we using it, we should at least mv it to a generic place such as utils_test.libvirt. But it seems impossible for this function, as it looks very specific for lgf test. So just building a command here is better I think.