libvirt: configuration element for virtual controller

While Libvirt can automatically configure a controller for most
of the devices, some need to have an explicit controller element
to be provided.

Partially implements blueprint libvirt-virtio-scsi-driver

Change-Id: Id0b6c597362cfb66af0d8c5d43f777e4fa3267fe
Merge "libvirt: configuration element for virtual controller"