Sets the default monitor for a VM, meaning that when a test accesses the monitor property of a VM class instance, that one monitor will be returned.
Usually a VM will have a single monitor, and that will be a regular Human monitor:
main_monitor = humanmonitor1
If a main_monitor is not defined, the monitor property of a VM class instance will assume that the first monitor set in the monitors list is the main monitor.
No other documentation currently references this configuration key.