libxl: add support for FreeBSD uuid implementation

libxl: add support for FreeBSD uuid implementation

Add the FreeBSD specific uuid implementation. Since uuid_t is not
defined as an array, but as a struct on FreeBSD, use a union with a
array in order to be able to return the uuid as a bytearray.

Also, added a libxl internal compile time assert macro, that is used
to assert that the size of uuid_t is the same as the union used in
libxl.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>