xenserver: Hoist identical bridge and vswitch functions into common code.

xenserver: Hoist identical bridge and vswitch functions into common code.

The previous commit made pif_bridge_name() in the bridge and vswitch
versions of interface-reconfigure functionally identical, so this commit
hoists them into a single common implementation in InterfaceReconfigure.py.

pif_is_bridged() also comes along for the ride because it is also generic
and because it is logically related.  Only the bridge code uses it at the
moment.

Suggested-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>