vbox: Add SAS controller support

vbox: Add SAS controller support

In VirtualBox SAS and SCSI are separate controller types whereas libvirt
does not make such distinction. This patch adds support for attaching
the VBOX SAS controllers by mapping the 'lsisas1068' controller model in
libvirt XML to VBOX SAS controller type. If VBOX VM has disks attached
to both SCSI and SAS controller libvirt domain XML will have two
<controller type='scsci'> elements with index and model attributes set
accordingly. In this case, each respective <disk> element must have
<address> element specified to assign it to respective SCSI controller.