Previously disk_bus values were never validated and could easily end up
being ignored by the underlying virt driver and hypervisor.
For example, a common mistake made by users is to request a virtio-scsi
disk_bus when using the libvirt virt driver. This however isn't a valid
bus and is ignored, defaulting back to the virtio (virtio-blk) bus.
This change adds a simple validation in the compute API using the
potential disk_bus values provided by the DiskBus field class as used
when validating the hw_*_bus image properties.
Closes-Bug: #1876301
Change-Id: I77b28b9cc8f99b159f628f4655d85ff305a71db8
(cherry picked from commit 5913bd889f)
fb31ae430a