"qemu-io reopen" doesn't allow changing the writethrough setting of
the cache, but the check is wrong, causing an error even on a simple
reopen with the default parameters:

   $ qemu-img create -f qcow2 hd.qcow2 1M
   $ qemu-system-x86_64 -monitor stdio -drive if=virtio,file=hd.qcow2
   (qemu) qemu-io virtio0 reopen
   Cannot change cache.writeback: Device attached

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>