hyper-v: Use UUID API for exporting the GUID

There is export_guid() function which exports guid_t to the u8 array.
Use it instead of open coding variant.

This allows to hide the uuid_t internals.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200422125937.38355-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Wei Liu <wei.liu@kernel.org>