docs: Introduce xenstore paths for PV driver information

docs: Introduce xenstore paths for PV driver information

For domain management purposes it is convenient to be able to see
information about PV drivers in xenstore. The XAPI toolstack in
XenServer has always created a ~/drivers path for this purpose.

This patch documents that path and also adds a specification of how
it should be used.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Tim Deegan <tim@xen.org>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>