libxl: Domain destroy: fork

libxl: Domain destroy: fork

Call xc_domain_destroy in a subprocess.  That allows us to do so
asynchronously, rather than blocking the whole process calling libxl.

The changes in detail:

 * Provide an libxl__ev_child in libxl__domain_destroy_state, and
   initialise it in libxl__domain_destroy.  There is no possibility
   to `clean up' a libxl__ev_child, but there need to clean it up, as
   the control flow ensures that we only continue after the child has
   exited.

 * Call libxl__ev_child_fork at the right point and put the call to
   xc_domain_destroy and associated logging in the child.  (The child
   opens a new xenctrl handle because we mustn't use the parent's.)

 * Consequently, the success return path from domain_destroy_domid_cb
   no longer calls dis->callback.  Instead it simply returns.

 * We plumb the errorno value through the child's exit status, if it
   fits.  This means we normally do the logging only in the parent.

 * Incidentally, we fix the bug that we were treating the return value
   from xc_domain_destroy as an errno value when in fact it is a
   return value from do_domctl (in this case, 0 or -1 setting errno).

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>
Tested-by: Jim Fehlig <jfehlig@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>