It's not possible to change the vdisk bus type to SCSI if machine type is set to Q35 during initial VM creation.   Reproducable: Create a VM with default machine type Q35 or set it manual. I tried the Fedora template and the Win10 template and switched to Q35  set size of the vdisk 1G or whatever you like choose a bus type except of SCSI untick "start VM after creation" everything else on default hit "create" Edit VM and switch bus type t