libxl: remove per-domain libxl_ctx

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>