Comment 13 for bug 1248025

This commit in libvirt-bin upstream may be useful for tracking this down:

commit 03ede2f69d294af49642805384d54ac8ac7a4adc
Author: Daniel Veillard <email address hidden>
Date: Fri Apr 1 19:30:53 2011 +0800

    Fix libxl driver startup

      When you happen to have a libvirtd binary compiled with the
    libxenlight driver (say you have installed xen-4.1 libraries)
    but not running a xen enabled system, then libvirtd fails to start.

    The cause is that libxlStartup() returns -1 when failing to initialize
    the library, and this propagates to virStateInitialize() which consider
    this a failure. We should only exit libxlStartup with an error code
    if something like an allocation error occurs, not if the driver failed
    to initialize.

    * src/libxl/libxl_driver.c: fix libxlStartup() to not return -1
      when failing to initialize the libxenlight library