cpus-common: nuke finish_safe_work

It was introduced in commit ab129972,
with the following motivation:

  Because start_exclusive uses CPU_FOREACH, merge exclusive_lock with
  qemu_cpu_list_lock: together with a call to exclusive_idle (via
  cpu_exec_start/end) in cpu_list_add, this protects exclusive work
  against concurrent CPU addition and removal.

However, it seems to be redundant, because the cpu-exclusive
infrastructure provides suffificent protection against the newly added
CPU starting execution while the cpu-exclusive work is running, and the
aforementioned traversing of the cpu list is protected by
qemu_cpu_list_lock.

Besides, this appears to be the only place where the cpu-exclusive
section is entered with the BQL taken, which has been found to trigger
AB-BA deadlock as follows:

    vCPU thread                             main thread
    -----------                             -----------
async_safe_run_on_cpu(self,
                      async_synic_update)
...                                         [cpu hot-add]
process_queued_cpu_work()
  qemu_mutex_unlock_iothread()
                                            [grab BQL]
  start_exclusive()                         cpu_list_add()
  async_synic_update()                        finish_safe_work()
    qemu_mutex_lock_iothread()                  cpu_exec_start()

So remove it.  This paves the way to establishing a strict nesting rule
of never entering the exclusive section with the BQL taken.

Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
Message-Id: <20190523105440.27045-2-rkagan@virtuozzo.com>