Here is the info i gathered about the vega. May not be accurate.
There is no working reset option and the vega will not survive the bus reset after initialized. If CSM is disabled, EFI boot is enabled for VGA or is the boot VGA vega is initialized by BIOS and will not survive bus reset.
Basically the vega does not support reset on the bus level but must be reset via PSP mode1 reset. The code exists in amdgpu driver but it would need to be ported to qemu as a quirk, if its even possible.
Workarounds:
https://gist.github.com/numinit/1bbabff521e0451e5470d740e0eb82fd this is the patch that will prevent the bus reset and vega can be booted and windows can be rebooted. Works also for pure EFI boot. Lets me pass the vega in first slot wired to NUMA1 as disabling CSM changes the boot order to loves port vga card.
Linux Host, Windows Guest, GPU passthrough reinitialization fix this describes how to setup windows so the VM can be shut down and started up. This simply forces windows to do the shut down and start up the GPU doing the reset for you. But it is no longer working in newest win10.
If the host crashes or the DevCon no longer works you need to do the whole suspend to RAM to reset the GPU or do a reboot.