Just wanted to post here to say I am having a very similar issue to what is being discussed here, and I wanted to find out what the status of this issue is.
The post about my issue is here: Windows 10 Questions - #4 by TalarusLuan
After thinking about it some more, I was wondering if it might have something to do with the fact that the passthrough adapter is hidden with both xen-pciback and rd.qubes, even though it is the primary boot adapter. Qubes does end up using the secondary adapter I have in the system, but the log still says the efifb is assigned to the primary card (despite also having video=efifb:off on the kernel cmdline).
I am getting all the same messages about the adapter that appear in this issue. The Video Shadow copy of the ROM is created in the address range as well.
If there is no news, would it be something that could be kicked up to the xen developers?
Thank you for any info and your consideration.