The main advantage of circular lists (the fact that the head node
has the same memory layout as any other node) is completely negated
by the implementation in qemu-queue.h.  Not surprisingly, nobody
uses QCIRCLEQ.  While this might change if RCU is ever adopted by
QEMU, the QLIST is also RCU-friendly and in fact it is used in a
RCU-like manner by 9pfs already.  So, just kill QCIRCLEQ.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>