libxl: create 'drivers', 'feature' and 'attr' xenstore paths

libxl: create 'drivers', 'feature' and 'attr' xenstore paths

My recent patch series 'docs: Document xenstore paths' included 3
patches documenting new xenstore paths to allow PV drivers/agents in
guests to advertise version information, significant features and
attributes (such as assigned IP addresses).

This patch adds the necessary code to libxl to create these paths in
xenstore when a domain is created.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>