Both Graphical and Telnet Console for QEMU VM

[ghost]

Some QEMU devices use both the telnet console and the graphical console. A prominent example are the ASAv, the images from the Cisco download site use the graphical console, while the images from Virl use the telnet console. Another one are the pfsense images, where you can switch the console.

Currently it's not easy to look on the other console type to see, if that was the right one. You have to stop the VM, reconfigure the console, start it again and hope the best.

With some help of the qemu additional settings it's already possible to have both consoles simultaneously, see https://www.b-ehlers.de/blog/2018/08/03/gns3-qemu-graphical-and-telnet-console.html. But that's only usable for experienced users.

Here my idea to make it simpler:
A qemu node gets an auxiliary console, which uses the other console type. If the main console is telnet, the auxiliary console will be VNC (or Spice?), if the main console is a graphical one, the auxiliary will be a telnet console. As the auxiliary console is mainly for debugging, the implementation can be simpler as the main console, some restrictions should be acceptable.

[grossmj]

PRs #3029 and GNS3/gns3-server#1804 opened.

[grossmj]

Implemented.