Old topic but figured I'd put this information here in case anyone else runs into a similar issue.
This fix only works for BIOS/Legacy boot not UEFI because UEFI doesn't use VGA directly. There's not a solution for UEFI at this point but there should be one when XCP-ng moves to Xen 4.17+ in 8.3 release.
Bit more information regarding the future potential fix is documented in xen commit 927aee5