hotplug: fix ip_of for systems using a peer-to-peer link

hotplug: fix ip_of for systems using a peer-to-peer link

This is from an old Debian bug at
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=437127

I'm slightly inferring what the configuration which trigger this issue is, the
bug report says:

    This code however fails if no slash is present in the address, which is the
    case in my configuration:

    $ ip addr show dev eth0
    2: eth0: <BROADCAST,MULTICAST,UP,10000> mtu 1500 qdisc pfifo_fast qlen 1000
        link/ether 00:16:17:90:b4:35 brd ff:ff:ff:ff:ff:ff
        inet 88.198.12.XXX peer 88.198.12.XXX/32 brd 88.255.255.255 scope global eth0

I hope either Marc or Stefan can correct me if I have surmised wrongly what
this configuration represents.

This function is used in the vif-route configuration.

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