Make sure git clone gets the right kernel branch

Make sure git clone gets the right kernel branch

When cloning kernel repo:
  1. make remote called "xen" rather than the default "origin"
  2. directly checkout the desired branch, rather than the default
  then the desired one

Git 1.5 doesn't support -b on git clone, and seems to do something odd
with the checkout branch argument, so avoid using the newer
commandline options.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>