core: Change NIC type from sPAPR VLAN to any other

After created the VM, added a network interface and have changed its
type, the VM cannot be started.

Change-Id: Ia77afccc75bd500db6f3b88872847a06ebe43513
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1060729
Signed-off-by: Leonardo Bianconi <leonardo.bianconi@eldorado.org.br>