Long ago I incorrectly associated libxl fd and timer registrations
with per-domain libxl_ctx objects.  When creating a libxlDomainObjPrivate,
a libxl_ctx is allocated, and libxl_osevent_register_hooks is called
passing a pointer to the libxlDomainObjPrivate.  When an fd or timer
registration occurred, the registration callback received the
libxlDomainObjPrivate, containing the per-domain libxl_ctx.  This
libxl_ctx was then used when informing libxl about fd events or
timer expirations.

The problem with this approach is that fd and timer registrations do not
share the same lifespan as libxlDomainObjPrivate, and hence the per-domain
libxl_ctx ojects.  The result is races between per-domain libxl_ctx's being
destoryed and events firing on associated fds/timers, typically manifesting
as an assert in libxl

libxl_internal.h:2788: libxl__ctx_unlock: Assertion `!r' failed

There is no need to associate libxlDomainObjPrivate objects with libxl's
desire to use libvirt's event loop.  Instead, the driver-wide libxl_ctx can
be used for the fd and timer registrations.

This patch moves the fd and timer handling code away from the
domain-specific code in libxl_domain.c into libxl_driver.c.  While at it,
function names were changed a bit to better describe their purpose.

The unnecessary locking was also removed since the code simply provides a
wrapper over the event loop interface.  Indeed the locks may have been
causing some deadlocks when repeatedly creating/destroying muliple domains.
There have also been rumors about such deadlocks during parallel OpenStack
Tempest runs.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>