Hi NeonFlash,
There is no such option as monitor.debugOnStartGuest64. You can use monitor.debugOnStartGuest32 to cause the VM to pause immediately at power-on and wait for a debugger on the 32-bit guest debugStub port. The same option is also effective for 64-bit guests, since the processor first powers on in 16-bit mode, which is handled through the 32-bit debugStub port.
But... There's no built-in mechanism to pause or break when the processor first starts 64-bit operation, and the starting point (the reset vector) is a long way away from the point you're interested in. If you want to break when the guest kernel switches to 64-bit mode, it should be possible to connect to the 32-bit debugStub and set a breakpoint at the address of the guest kernel code that switches to 64-bit mode, or even in the early 64-bit code. When the VM hits the breakpoint, you can attach your 64-bit gdb to the 64-bit debugStub and continue debugging.
(Switching back and forth between the 32-bit and 64-bit debugStubs can be... funky... at times.)
Cheers,
--
Darius