qemu-img: let 'qemu-img convert' flush data

The 'qemu-img convert -h' advertise that the default cache mode is
'writeback', while in fact it is 'unsafe'.

This patch 1) fix the help manual and 2) let bdrv_close() call bdrv_flush()

2) is needed because some backend storage doesn't have a self-flush
mechanism(for e.g., sheepdog), so we need to call bdrv_flush() to make
sure the image is really writen to the storage instead of hanging around
writeback cache forever.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>