This converts the libvirt VIF handler class to call out to
os-vif for plugging/unplugging the Linux Bridge and OpenVSwitch
VIF types.
This should provide identical functionality that which exists
today, allowing seemless upgrade. The new os-vif plugins have
some config parameters in a new section, however, the os-vif
config params have suitable deprecated annotations to ensure
they will pull values from the old names/groups in the config
file.
Blueprint: os-vif-library
Change-Id: Ib951680917a3cda8c44d237c63d3ef2c09aa0057