Allow network interface type to be specified

This change also fixes the following error on modern RHEV/oVirt environments:
ERROR vagrant: #<VagrantPlugins::OVirtProvider::Errors::AddInterfaceError: Error while adding new interface to VM. rhevm is not a member of NicInterface. Possible values for NicInterface are: e1000, virtio, rtl8139, rtl8139_virtio, spapr_vlan>
ERROR vagrant: Error while adding new interface to VM. rhevm is not a member of NicInterface. Possible values for NicInterface are: e1000, virtio, rtl8139, rtl8139_virtio, spapr_vlan