libxl: remove per-domain libxl_ctx
    
    Although needed in the Xen 4.1 libxl days, there is no longer any
    benefit to having per-domain libxl_ctx.  On the contrary, their use
    makes the code unecessarily complicated and prone to deadlocks under
    load.  As suggested by the libxl maintainers, use a single libxl_ctx
    as a handle to libxl instead of per-domain ctx's.
    
    One downside to using a single libxl_ctx is there are no longer
    per-domain log files for log messages emitted by libxl.  Messages
    for all domains will be sent to /var/log/libvirt/libxl/libxl-driver.log.
    
    Signed-off-by: Jim Fehlig <jfehlig@suse.com>