hw/xen_disk: ioreq not finished on error

Bug fix: routines 'ioreq_runio_qemu_sync' and 'ioreq_runio_qemu_aio'
won't call 'ioreq_unmap' or 'ioreq_finish' on errors, leaving ioreq in
the blkdev->inflight list and a leak.

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>