CA-99179: Patches XAPI to allow clients to stop XAPI from generating
a udhcpd config file that includes advertising dom0's IP as
the default gateway for the network.

When using the host internal management network with Windows VMs
the network adapter will be allocated a high metric based on its speed,
and so traffic to unkown destinations may be routed via Dom0.

Signed-off-by: Rob Dobson <rob@rdobson.co.uk>
CA-99179: Adding an API for configuring the udhcp server in Dom0.

Signed-off-by: Rob Dobson <rob@rdobson.co.uk>