libxl: Do not call assert() in signal handlers

libxl: Do not call assert() in signal handlers

assert is not async-signal-safe.

In practice the effect of calling assert there is that if the
assertion fails we might get a secondary crash, or other undesirable
behaviour from stdio (which is how assert usually reports failures).

Mention in a comment in libxl__self_pipe_wakeup that it has to be
async-signal-safe.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>