[Windows 10 VM only starts with 1 core.]

Try removing 

 

 

remove it shuld be there but isen't working for me in unraid 6.8.3
<hyperv>
      <relaxed state='on'/>
      <vapic state='on'/>
      <spinlocks state='on' retries='8191'/>
      <vendor_id state='on' value='none'/>
    </hyperv>

and sett hypervclock to 'no'
<timer name='hypervclock' present='no'/>

works for me don't know way

[Windows 10 Gaming VM with Nvidia]

I think i might know the problem isent unraid its hardware / BIOS.

 

Display Port cabel have a bad habit of making BOOT problems.

The system draw power true dp cabel and shorcut bios from booting and it only apers after shutdown/reboot.

On real cold bot it offen works when power is total depleeted.

It tock me 4 days to find out way my pc dident bot untill i removed PD cabel.

I contacted my Mainbord manufactory and thy said i had to flach bios ovh try changing boot mode.

In the end i just change to HDMI problem solved