libxl: Open code rw and ro node creation.

libxl: Open code rw and ro node creation.

Use a new libxl__xs_mkdir() to do this and also clean up extraneous
node creation while in the neighbourhood. Checking 'xenstore-ls -fp'
output before and after shows that, as well as the disappearance of
error, drivers, messages and domid, the following perms change is also
present:

-device/suspend = ""   (ndomU)
+device/suspend = ""   (n0,rdomU)

I believe the new perms are more desirable than the old ones.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>