ipmi: Use defined constant for UUID representation

Instead of magic number use pre-defined constant for UUID binary and
string representations.

While here, drop the implementation details of guid_t type.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[Also converted a "17" in the error string to UUID_SIZE + 1]
Signed-off-by: Corey Minyard <cminyard@mvista.com>