xenserver: Fix bringing up VLAN PIFs.

xenserver: Fix bringing up VLAN PIFs.

In configure_datapath(), bringing up a VLAN PIF's datapath is no different
from bringing up a non-VLAN PIF's datapath, but the code was making
unnecessary distinctions.  Also, the test
	if parent and datapath:
did not work at all, because there was no variable named 'datapath' in
scope and no obvious source of one.

In preconfigure(), it then becomes necessary to create a VLAN bridge for a
VLAN PIF, in a straightforward way.

Signed-off-by: Ben Pfaff <blp@nicira.com>