Add multiscreen mode

[MatejSpindler]

This commit adds max_outputs parameter to the graphic device if max_outputs option was provided in the .conf file of the VM.

Example:

guest_os="xx"
disk_img="xx.qcow2"
max_outputs="2"

This option only reliably works with spice-app as the display (#472).

Other display options:

• sdl is crashing on qemu 7.0, could be an archlinux issue,
• gtk I can't find options to open 2 windows and mouse coordinates are messed up,
• spice I can't find options to open 2 windows,