xen/timers: Fix memory leak with cpu unplug/plug (take 2)

xen/timers: Fix memory leak with cpu unplug/plug (take 2)

Previous attempts to fix this leak didn't identify the root cause, and
ultimately failed.  The cause is actually the CPU_UP_PREPARE case
(re)initialising ts->heap back to dummy_heap, which leaks the previous
allocation.

Rearrange the logic to only initialise ts once.  This also avoids the
redundant (but benign, due to ts->inactive always being empty) initialising of
the other ts fields.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>