libxl: Per-domain data storage for the convenience of the library user

libxl: Per-domain data storage for the convenience of the library user

We provide a mechanism whereby a user of the libxl library is able to
store some information alongside the domain.  The information stored
is a block of bytes.  Its lifetime is that of the domain - ie the
userdata is garbage collected alongside the domain if the domain is
destroyed.  (This is why the feature needs to be in libxl and cannot
be implemented in the user itself or in libxlutil.)

If a libxl caller does not need to use this feature it can ignore it.

The data is tagged with the (self-declared) name of the libxl user, so
that different users cannot accidentally trip over each others'
userdata.  The data is not interpreted at all by libxl.

To assist developers and people debugging, there is a registry of the
known userdata userids, and the corresponding data format as declared
by that libxl user, in libxl.h next to these declarations:

 int libxl_userdata_store(struct libxl_ctx *ctx, uint32_t domid,
                               const char *userdata_userid,
                               const uint8_t *data, int datalen);
 int libxl_userdata_retrieve(struct libxl_ctx *ctx, uint32_t domid,
                                  const char *userdata_userid,
                                  uint8_t **data_r, int *datalen_r);

The next patch will introduce the data for the userid "xl".

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>