Datamodel: add a `default_message` value
Datamodel: add a `default_message` value

Users of the datamodel (I'm looking at you, xen-api-sdk) can construct
message values using

  { default_message with
      my_field = "";
      my_other_field = ""; }

When the record is extended in future, the default value will then be
used without needing a code change.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

euanh/xen-api,vasilenkomike/xen-api,guard163/xen-api,robertbreker/xen-api,agimofcarmen/xen-api,jjd27/xen-api,srowe/xen-api,simonjbeaumont/xen-api,vasilenkomike/xen-api,robertbreker/xen-api,rafalmiel/xen-api,jjd27/xen-api,djs55/xen-api,guard163/xen-api,agimofcarmen/xen-api,anoobs/xen-api,simonjbeaumont/xen-api,anoobs/xen-api,Frezzle/xen-api,guard163/xen-api,salvocambria/xen-api,robertbreker/xen-api,anoobs/xen-api,anoobs/xen-api,euanh/xen-api,Frezzle/xen-api,rafalmiel/xen-api,rafalmiel/xen-api,robertbreker/xen-api,thomassa/xen-api,salvocambria/xen-api,guard163/xen-api,simonjbeaumont/xen-api,djs55/xen-api,djs55/xen-api,agimofcarmen/xen-api,anoobs/xen-api,srowe/xen-api,djs55/xen-api,thomassa/xen-api,agimofcarmen/xen-api,simonjbeaumont/xen-api,rafalmiel/xen-api,jjd27/xen-api,thomassa/xen-api,Frezzle/xen-api,simonjbeaumont/xen-api,jjd27/xen-api,thomassa/xen-api,rafalmiel/xen-api,salvocambria/xen-api,vasilenkomike/xen-api,srowe/xen-api,guard163/xen-api,djs55/xen-api,robertbreker/xen-api,anoobs/xen-api,agimofcarmen/xen-api,euanh/xen-api,salvocambria/xen-api,Frezzle/xen-api,vasilenkomike/xen-api,salvocambria/xen-api,thomassa/xen-api,djs55/xen-api,salvocambria/xen-api,Frezzle/xen-api,jjd27/xen-api,vasilenkomike/xen-api,agimofcarmen/xen-api,euanh/xen-api,euanh/xen-api,thomassa/xen-api,euanh/xen-api,simonjbeaumont/xen-api,rafalmiel/xen-api,robertbreker/xen-api,vasilenkomike/xen-api,srowe/xen-api,srowe/xen-api,jjd27/xen-api,guard163/xen-api