There are already breakpoints for both VCPU and Native.
Press CTRL-F6 -> Press CTRL-D
Click on any instruction, (the red X clears all breakpoints, CTRL-M switches between VCPU and Native once you have pressed CTRL-D).
You can also add breakpoints to your VCPU ASM code with _breakpoint_, just use it as if it was a label next to the instruction you want to break on.
When you load that source file into the emulator, the Debugger will automatically start in breakpoint mode.
CTRL-F7 runs to next breakpoint, CTRL-F8 single steps, CTRL-F9 runs until watch variables changes, it's all in the help screen.
There are a few quirks, it's a work in progress, i.e. it can be hard to break on code waiting on input, debugger can timeout, but right now it mostly works.