You found the issue!
VirtIOSCSI can only do its magic, when it's actually used. And once the boot disks was
running using AHCI emulation it's a little hard to make it "re-attach" to
SCSI.
I am pretty sure it could be done, like you could make Windows disks switch from IDE to
SATA/AHCI with a bit of twiddling in the registry using recovery mode.
I think I managed to feed the Windows installer a floppy image with the VirtIO drivers at
one point, but it took me half a workday I think...
That is one of the reasons I like importing my Windows VMs for oVirt from VirtualBox. And
it doesn't seem to like the OVAs VMware produces.