To make the virtual appliance work full blown, you need an RE and PFE. The cosim image is just the PFE part of it, you should also be running "jinstall-vqfx-10-f-18.4R1.8.qcow2" and ensure there is a dedicated virtual connection (em1 typically) between the RE and PFE. On PFE you can check if the interface to RE is up (typically eth1). Once the RE to PFE connection is up, we can just login to the RE to get to Junos CLI, you won't need to login to the PFE.
Check this for some basic troubleshooting for this RE/PFE connection: