Pass on the destination host in StartCommand

I added this in commit bc94948e0604e0e5931759be3c3d3155e84686f6 to be able to bind
the VNC on KVM on the Private IP Address of the Hypervisor.

This got (accidentally) reverted in commit 110903a91a21c04b931a26354a04bd7f534ba050 breaking
this behaviour with KVM.

By passing the destination host again in StartCommand we are able to bind the VNC to the private
IP address of the hypervisor.

This makes sure the VNC is not open for the world and users don't have to firewall these ports, nor
do they have to change "vnc_listen" in their qemu.conf libvirt settings.