thread-pool: clean up thread_pool_completion_bh()

This patch simplifies thread_pool_completion_bh().

The function first checks elem->state:

  if (elem->state != THREAD_DONE) {
      continue;
  }

It then goes on to check elem->state == THREAD_DONE although we already
know this must be the case.

The QLIST_REMOVE() is duplicated down both branches of an if-else
statement so that can be lifted out as well.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1427992762-10126-1-git-send-email-stefanha@redhat.com
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
