Add ibmveth model as a supported network driver for KVM

ibmveth (spapr-vlan) is supported by KVM/Qemu on ppc64. This change
allows users to change hw_vif_model='spapr-vlan' property to use
ibmveth driver.

Change-Id: Ibec6abad458c5cb15df57afa05f7f0742e4feb14
Merge "Add ibmveth model as a supported network driver for KVM"