Add support and checks for version in LUXI

A new constant, LUXI_VERSION, is used to verify the peer's version. The
version is optional, so old(er) clients and servers talking to peers not
supporting it won't break. Example with mismatching library:

$ gnt-instance list
Unhandled Ganeti error: LUXI version mismatch, server 2020000, request
1010000

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>