Q35 switch vDisk BUS to SCSI - double use of PCI Address error

It's not possible to change the vdisk bus type to SCSI if machine type is set to Q35 during initial VM creation.

Reproducable:

1. 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 
2. set size of the vdisk 1G or whatever you like
3. choose a bus type except of SCSI
4. untick "start VM after creation"
5. everything else on default
6. hit "create"
7. Edit VM and switch bus type to SCSI
8. hit "Update"

Using i440fx during initial VM creation doesn't show that error. You can switch around the bus types without any errors. The other way around by creating a Q35 SCSI disk on initial setup of the VM and switching it afterwards works fine. Switching from Q35 to i440fx produces another error. No matter if a vdisk is created or attached during first creation or no disk is configured. I'm pretty sure that worked in an earlier version.