libxl: remove list of timer registrations from libxlDomainObjPrivate

libxl: remove list of timer registrations from libxlDomainObjPrivate

Due to some misunderstanding of requirements libxl places on timer
handling, I introduced the half-brained idea of maintaining a list
of timeouts that the driver could force to expire before freeing a
libxlDomainObjPrivate (and hence libxl_ctx).  But testing all
the latest versions of Xen supported by the libxl driver (4.2.3,
4.3.1, 4.4.0 RC3), I see that libxl will handle this just fine and
there is no need to force expiration behind libxl's back.  Indeed it
may be harmful to do so.

This patch removes the timer list, allowing libxl to handle cleanup
of its timer registrations.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>