tools/hotplug: Do not mind if "ip link set" fails on vif backends

tools/hotplug: Do not mind if "ip link set" fails on vif backends

Most versions of netback do not support setting the MAC address.  This
means that c/s 937488219719 causes the hotplug script to break for
vifs, even as it fixes the bridge-wrong-MAC-address problem for tap
devices (used for emulated nics).

The mac-setting operation is not necessary for vifs since they are
hardcoded to fe:ff* anyway.

As a band-aid, add "|| true" to the call to ip link, so that this
error does not cause the hotplug operation to fail.  There will still
be an error message printed until we can fix this better.

Suggested-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>