Github user nathanejohnson commented on the issue:
https://github.com/apache/cloudstack/pull/1955
Also, I just made a commit, I have not squashed it because I wanted to get
some feedback. I noticed that vmware uses details to pass rootDiskController
for the bus type, and there was even some code that inspected this in the kvm
plugin. With this latest commit, it will actually honor either OS type "Other
PV Virtio-SCSI" or rootDiskController passed in via details, with details
overriding OS type. I also made a minor tweak to the UI to allow setting the
root disk bus via the add template dialog in the UI:

So this way you can select virtio scsi by one of the following three ways:
1) selecting os type as "Other PV Virtio-SCSI (64 bit)"
2) uploading a template and specifying details[0].rootDiskController=scsi,
either from the UI or the API, then creating a VM from this template
3) deploying a VM and specifying details[0].rootDiskController=scsi ,
currently only available via the API
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