Fix find_kpgd_method0 for live VMs

This is an attempt to fix an issue during init of a live VM where PsActiveProcessHead can be resolved by kdbg but PsInitialSystemProcess can't, where the EPROCESS at PsActiveProcessHead is not the system process.

The test setup I have for this is:
1) a paused VM where PsActiveProcessHead is not the System process
2) a memory snapshot of the VM at that time

For the live VM, the values being used for win_sysproc/kpgd by kpgd_method0 are for a current process that is not the System process.  For 2, find_kpgd_method2 is used to brute force scan for the System process, which results in the correct values.

I'm trying to remember why we do this here.. You say PsActiveProcessHead does not point to the System process, but couldn't it be that we actually just skip it here by reading the next pointer?