libxl: replace the usage of uuid_t with a char array

libxl: replace the usage of uuid_t with a char array

The internals of the uuid_t struct don't match a big endian octet stream on
*BSD systems, which means that it cannot be directly casted to a
uint8_t[16].

In order to solve that change the type to be an unsigned char[16], which
doesn't imply any other change on Linux. On *BSDs change the helpers so that
the uuid is always stored as a big endian byte stream.

NB: tested on FreeBSD and Linux only.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Discussed-with: Ian Jackson <Ian.Jackson@eu.citrix.com>
Discussed-with: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>