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.

While I think the kpgd is zeroed out if the os_interface is not initialized, it would be easier to understand what happens under different init modes with this function if its also checked here.