libxl: New utility functions in for reading and writing files.

libxl: New utility functions in for reading and writing files.

We introduce these functions in libxl_utils.h:

  int libxl_read_file_contents(struct libxl_ctx *ctx, const char
  *filename,
                               void **data_r, int *datalen_r);
  int libxl_read_exactly(struct libxl_ctx *ctx, int fd, void *data,
  ssize_t sz,
                         const char *filename, const char *what);
  int libxl_write_exactly(struct libxl_ctx *ctx, int fd, const void
  *data,
                          ssize_t sz, const char *filename, const char
  *what);

They will be needed by the following patches.  They have to be in
libxl.a rather than libxutil.a because they will be used, amongst
other places, in libxl itself.

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