docs: Introduce xenstore paths for hotplug features

Without some indication from a guest it is not possible for a
toolstack to know whether instantiation of a new vbd or vif should
result in a new PV device of the appropriate type being brought online.
(In other words whether guest PV drivers are present and functioning).

This patch documents two paths which vif and vbd frontend drivers can
use to advertise their ability to respond to new vif or vbd
instantiations.

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>

docs/misc/xenstore-paths.markdown

@@ -409,6 +409,18 @@ A domain writable path. Available for arbitrary domain use.
 A domain may write information about installed PV drivers using
 paths of this form.
 
+#### ~/feature/hotplug/vif = ("0"|"1") [w]
+#### ~/feature/hotplug/vbd = ("0"|"1") [w]
+
+By setting these paths to "1" a guest can indicate to a toolstack
+that it is capable of responding immediately to instantiation of,
+respectively, new vif by bringing online a new PV network device or
+a new vbd by bringing online a new PV block device.
+If the guest sets this path to "0" then it is indicating that it is
+definitely unable to respond immediately and hence the toolstack should
+defer instantiaton to the next VM start. However, if the path is absent
+then the toolstack may attempt the operation.
+
 ### Paths private to the toolstack
 
 #### ~/device-model/$DOMID/state [w]