core, engine: sPAPR VLAN support

This adds support for a ppc64-specific network interface called sPAPR
VLAN. There are changes only in the VmInterfaceType Enum and a small
refactoring in the code to allow the interface type passed to libvirt
to be different from the Enum name.

Change-Id: I12b9d760631200a9ae6508ecceb150854b2e9e6b
Signed-off-by: Vitor de Lima <vitor.lima@eldorado.org.br>