tools: Introduce a xc_xenver_buildid() wrapper

... which converts binary content to hex automatically.

Update libxl to match.  No API/ABI change.

This removes a latent libxl bug for cases when the buildid is longer than 4092
bytes.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>

tools/include/xenctrl.h

@@ -1581,6 +1581,7 @@ char *xc_xenver_extraversion(xc_interface *xch);
 char *xc_xenver_capabilities(xc_interface *xch);
 char *xc_xenver_changeset(xc_interface *xch);
 char *xc_xenver_commandline(xc_interface *xch);
+char *xc_xenver_buildid(xc_interface *xch);
 
 int xc_flask_op(xc_interface *xch, xen_flask_op_t *op);