xencommons: move module list into a generic place

xencommons: move module list into a generic place

This will allow us to share the same module list with
systemd, and lets us upkeep it in one place. Document this
while at it on the top level README and expand on the wiki:

http://wiki.xen.org/wiki/Category:Host_Configuration#Kernel_modules

In order to upkeep parallelism builds be explicit about the
requirement to complete all actions before any installation
targets.

Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>