Update existing hotplug documentation to state that virtio-block and virtio-scsi bus types are supported. Depending on the implementation we will need to state that there are limitations to the number of virtio-block disks that can be hotplugged. If an API is added to adjust this limit then that should also be documented.

User stories

• As a VM owner, I would like not to care about the bus type. I'd like to create VMs that have both good performance and hotpluggability by default.
• I have a Windows VM running on VMware, where I can hot-plug disks to it. I would like to to keep this functionality after migrating it to CNV. MTV's virt-v2v cannot convert Windows VM to virtio-scsi bus, so I'd like to virtio-blk (CNV's default) to support hot plug.
• I have a Linux VM running on VMware, where I can hotplug disks to it. MTV's virt-v2v converts its disks to virtio-blk devices named vd*, which do not currently support hotplug.