qemu-queue: drop QCIRCLEQ

qemu-queue: drop QCIRCLEQ

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>