To ensure that a newly created virtual machine has access to persistent storage you must create and attach a disk.

This Ruby example creates and attaches a virtual storage disk to a virtual machine.

# Locate the virtual machines service and use it to find the virtual
# machine:
vms_service = connection.system_service.vms_service
vm = vms_service.list(search: 'name=myvm')[0]

# Locate the service that manages the disk attachments of the virtual
# machine:
disk_attachments_service = vms_service.vm_service(vm.id).disk_attachments_service

# Use the "add" method of the disk attachments service to add the disk.
# Note that the size of the disk, the provisioned_size attribute, is
# specified in bytes, so to create a disk of 10 GiB the value should
# be 10 * 2^30.
disk_attachment = disk_attachments_service.add(
  OvirtSDK4::DiskAttachment.new(
    disk: {
      name: 'mydisk',
      description: 'My disk',
      format: OvirtSDK4::DiskFormat::COW,
      provisioned_size: 10 * 2**30,
      storage_domains: [{
        name: 'mydata'
      }]
    },
    interface: OvirtSDK4::DiskInterface::VIRTIO,
    bootable: false,
    active: true
  )
)

# Wait until the disk status is OK:
disks_service = connection.system_service.disks_service
disk_service = disks_service.disk_service(disk_attachment.disk.id)
loop do
  sleep(5)
  disk = disk_service.get
  break if disk.status == OvirtSDK4::DiskStatus::OK
end

For more information, see http://www.rubydoc.info/gems/ovirt-engine-sdk/OvirtSDK4/DiskAttachmentsService:add.