cpus: Let vm_stop[_force_state]() always flush block devices
Even if the VM is already stopped, we cannot assume that all data has
already been successfully flushed to disk. The flush during the previous
vm_stop() could have failed.
Run bdrv_flush_all() unconditionally so that we get an error each time
if the block device isn't really flushed.
Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>