hotplug: add openvswitch script

hotplug: add openvswitch script

Based on Waldi's RFC at
http://lists.xen.org/archives/html/xen-devel/2012-09/msg00943.html

To use it set vif.default.script="vif-openvswitch" in /etc/xen/xl.conf or use
script=vif-openvswitch in the vif configuration.

Appears to do the right thing for PV and HVM guests (including tap devices)
and with stubdomains.

In order to support VLAN tagging and trunking the "bridge" specified in the
configuration can have a special syntax, that is:

	BRIDGE_NAME[.VLAN][:TRUNK:TRUNK]

e.g.
- xenbr0.99
     add the VIF to VLAN99 on xenbr0
- xenbr0:99:100:101
     add the VIF to xenbr0 as a trunk port receiving VLANs 99, 100 & 101

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Bastian Blank <waldi@debian.org>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: dev@openvswitch.org