ovs-bugtool: Avoid running ethtool on non-physical devices.

There can be possibilities where there are hundreds of OVS
internal devices. In such a situation, running ovs-bugtool
can take a very long time to complete as multiple ethtool
commands are run on each interface in /sys/class/net. Once
the ovs-bugtool completes, most of the ethtool command outputs
would be incomplete with "timeouts" as we only give 30 seconds
for CAP_NETWORK_STATUS.

With the following patch, we only run ethtools on those interfaces
that have an associated "device". All physical interfaces have
this entry in /sys/class/net/${interface_name}/. Virtual interfaces
can have this entry too, if it has an underlying virtual device.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>