tools/hotplug/Linux: supply --physdev-is-bridged in iptables runes

tools/hotplug/Linux: supply --physdev-is-bridged in iptables runes

With newer (pvops) kernels logs get flooded with this iptables
warning: physdev match: using --physdev-out in the OUTPUT, FORWARD and
POSTROUTING chains for non-bridged traffic is not supported anymore

Using the --physdev-is-bridged option prevents this.
See also: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=571634#10

Signed-off-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>