GUID byte ordering swapped

[reicheratwork]

In the initialization of the ddsi_guid_prefix in the rtps_init function, the ordering of the bytes in the GUID does not follow that of the specification (e.g. the vendor ID should be first two bytes).
This is due to the different manners of addressing the bytes, in 4 byte units (as the union branch u) or as 1 byte units (as the union branch s), which causes an implicit byte swap being done.

The GUID should have the vendor ID (which for cyclonedds should be 0x01, 0x10) as the bytes 1/16 and 2/16, and 13-15/16 should be a counter of the lokal entities created.
E.g.: one of the GUIDs retrieved from a builtin topic 2d 61 10 01 9a 27 b5 b0 26 43 4d 2f 00 00 01 02

Looking at this, it seems that the bytes for the vendor id are swapped and at the 3rd and 4th bytes.