qemu-io: delete bs instead of leaking it

Using bdrv_close() is not enough to free a BlockDriverState.  Since we
explicitly create it with bdrv_new(), use bdrv_delete() to close and
delete it.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
