hw/xen_disk: aio_inflight not released in handling ioreq when nr_segments==0

In hw/xen_disk.c, async writing ioreq is leaked when
ioreq->req.nr_segments==0, because `aio_inflight` flag is not released
properly (skipped by misplaced "break").

Signed-off-by: Feiran Zheng <famcool@gmail.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
