docs: Introduce xenstore paths for PV control features

docs: Introduce xenstore paths for PV control features

XenServer already makes use of ~/control/feature-suspend being written
to advertise guest capability of responding to 'suspend' when written to
~/control/shutdown and, since they are derived from XenServer drivers,
the Xen Project Windows PV drivers attempt to write this value. The write
currently fails for libxl provisioned VMs because ~/control is read-only
to the guest (only ~/control/shutdown is writable, for ackowledgement
purposes).

This patch documents feature-suspend and also a set of similar control
feature flags, so that that they may be added to libxl provisioned
guests by subsequent patches:

feature-poweroff: PV drivers/agent can shut down the guest
feature-reboot: PV drivers/agent can reboot the guest
feature-s3: PV drivers/agent can trigger guest sleep (HVM only)
feature-s4: PV drivers/agent can trigger guest hibernate (HVM only)

The patch (bacause it adds features relating to S3 and S4 power states)
also clarifies that the initial set of platform properties mentioned are
booleans, and updates the specifier accordingly.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Acked-by: 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>