libxl: Fix error path in libxl_device_events_handler

libxl: Fix error path in libxl_device_events_handler

libxl_device_events_handler would fail to call AO_ABORT if it failed;
instead it would simply return rc.  (This leaves the egc etc. from the
now-abolished stack frame potentially live, and leaves the ctx
locked.)

In xl, this is of no consequence, because xl will immediately exit in
this situation.  This is very likely to be true in any other callers
(of which we don't know of any, anyway).

Coverity-ID: 1181840
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
CC: coverity@xenproject.org