webadmin: [Fix] Add sPAPR VSCSI in the Add Virtual Disk dialog

The "Add Virtual Disk" dialog was missing one of the supported disk
interfaces, the sPAPR VSCSI. This change adds this type to the
drop-down box.

Change-Id: I9837e0b18eaccbc70fed8fa45b5671c2e61abee2
Signed-off-by: Vitor de Lima <vitor.lima@eldorado.org.br>