Update to new xentore interface in v1.2.3

We use 'immediate' rather than 'with_xs' and 'transaction'
instead of 'with_xst'

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #4 from djs55/update-xenstore-api

Update to new xentore interface in v1.2.3