Remove unused Vifctl.vifctl and Vifctl.set_vif_name. Remove the bridge and

Remove unused Vifctl.vifctl and Vifctl.set_vif_name.  Remove the bridge and
antispoof parameters from Vifctl.network -- these are already handled by the
script, so it is redundant to have a separate override for these parameters
coming from outside.  Don't use xen.util.process.runscript, because we don't
need to return the output, and therefore can just use os.spawnl.

Remove XendRoot.get_vif_antispoof, matching change above.

Prefix XendRoot.network_script_dir onto the return value for
XendRoot.get_network_script(), removing that burden from callers.

Signed-off-by: Ewan Mellor <ewan@xensource.com>