Increase the static constant default number of loopback interfaces, nloopbacks,
to 8. These interfaces are very cheap, so there's no problem having a few
spare. Anyone with more than 8 NICs can still use the kernel command line or
the module parameter line to pass a larger limit to this module.

Detail the kernel command line parameter for nloopbacks in the diagnostic in
network-bridge.

Closes bug #381.

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