CoQueue do not need to remove any element but the head of the list;
processing is always strictly FIFO.  Therefore, the simpler singly-linked
QSIMPLEQ can be used instead.
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>