libxl: a domain can be dying but not shutdown

libxl: a domain can be dying but not shutdown

The shutdown code is only present if the domain is shutdown.
If we attempt to extract it from the flags from a dying but not
shutdown domain then we get values like '255' which is not a
valid LIBXL_SHUTDOWN_REASON_. We should use LIBXL_SHUTDOWN_UNKNOWN
in this case.

Signed-off-by: David Scott <dave.scott@citrix.com>
Acked-by: Rob Hoes <rob.hoes@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- updated comment in libxl_types.idl to match ]