For anyone who finds this in the future, I ran into a similar issue today when passing through a LSI Card,
Coincidentally on Address 05:00.0, also coincidentally on a Supermicro Board (mine is X10DRH-CT, OPs was a X8DA8)
I was originally writing a comment here to see if a solution had ever been found, but while taking/marking up screenshots of my Motherboards System Block Diagram, I discovered my problem...
Human Error of course.
Long story short, I added a PCI to 2xM.2 card, and in my BIOS, I bifurcated the wrong CPU/PCI Port.
Instead of Bifurcating the M.2 Card, I accidentally Bifurcated the LSI Card, which caused the Unknown Header Error AND since there was an unknown header, the VM Would throw the "Device or Resource is busy" error when trying to start.
Correcting the Bifurcation fixed the issue in my case.
Not sure if that was the issue in OPs case, but this might help someone else who stumbles across this
