Break common network code in xen-network-common.sh, and common vif hotplugging

Break common network code in xen-network-common.sh, and common vif hotplugging
code into vif-common.sh.

Fix the antispoofing in network-bridge to allow forwarding from physical
device to bridge, and vif0.0 to eth0.

Change the interface between netback driver and vif hotplugging scripts.  The
mac address and bridge details are read from the store directly by the script,
so that the kernel driver does not need to get involved.

Remove an awful lot of cruft from vif-bridge.  Move the iptables handling into
vif-common.sh, and fix it so that the new vif can get past the antispoofing
entries.  Merge vif-nat and vif-route into this structure too.

Use ip instead of ifconfig to get necessary details.

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