virt: acrn: Mark the uuid field as unused

After the commits for userspace (see Link tags below) the uuid field
is not being used in the ACRN code. Update kernel to reflect these
changes, i.e. do the following:
- adding a comment explaining that it's not used anymore
- replacing the specific type by a raw buffer
- updating the example code accordingly

The advertised field confused users and actually never been used.
So the wrong part here is that kernel puts something which userspace
never used and hence this may confuse a reader of this code.

Note, that there is only a single tool that had been prepared a year
ago for these forthcoming changes in the kernel.

Link: https://github.com/projectacrn/acrn-hypervisor/commit/da0d24326ed6
Link: https://github.com/projectacrn/acrn-hypervisor/commit/bb0327e70097


Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20221116162956.72658-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>