Call plug_vifs() for all instances in init_host

Remove the assumption that we are using the "flat" networking model
and there is a single bridge interface for all instances on a given
compute node.  Instead simply call plug_vifs() for each instance in
init_host.

Fixes bug 1083784

Change-Id: I4b367deb6851c06c80a4bdf0bd806cf5e867bb0a
(cherry picked from commit 77e38b7cfc2206ef4ac528d3c9faaa09adbf7f58)
Merge "Call plug_vifs() for all instances in init_host" into stable/folsom