Flush the addresses from the new eth0 before trying to add the new address to

Flush the addresses from the new eth0 before trying to add the new address to
it, when using ip on the kernel command line and not using the installed ifup /
ifdown.  This stops the script bailing out if there is already an address
attached to that device, which was leaving the device without routing (since
this has already been removed).

Fix the stop action under the same circumstances.  No attempt was made to
restore the kernel command line details to the interfaces if the ifdown failed
in that case.

Signed-off-by: Ewan Mellor <ewan@xensource.com>