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