docs: add PVH specification

docs: add PVH specification

Introduce a document that describes the interfaces used on PVH. This
document has been designed from a guest OS point of view (i.e.: what a guest
needs to do in order to support PVH).

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Mukesh Rathor <mukesh.rathor@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Jan Beulich <JBeulich@suse.com>
Cc: Mukesh Rathor <mukesh.rathor@oracle.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>