libxl: create xenstore nodes for control/feature-XXX flags

libxl: create xenstore nodes for control/feature-XXX flags

The xenstore-paths documentation specifies various control/feature-XXX
flags to allow a guest to tell a toolstack about its abilities to
respond to values written to control/shutdown. However, because the
parent control xenstore key is created read-only to the guest, unless
empty nodes for the feature flags are also created reat/write by the
toolstack, the guest will not be able to set any flags.

This patch adds code to create all specified feature flag nodes at
domain creation time.

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