vswitch: do not access XenAPI from VIF hotplug script

vswitch: do not access XenAPI from VIF hotplug script

XenAPI accesses must go through the pool master which introduces a
scalability issue.  All but one of the required values are already
present in xenstore for this reason

The only exception is the network UUID for an internal network. Rather
than working around the lack of callout to interface-reconfigure for
these networks simply pass the network uuid when creating calling out
to create the bridge.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>