io: fsync before closing a file channel

Make sure the data is flushed to disk before closing file
channels. This is to ensure data is on disk and not lost in the event
of a host crash.

This is currently being implemented to affect the migration code when
migrating to a file, but all QIOChannelFile users should benefit from
the change.

Reviewed-by: "Daniel P. Berrangé" <berrange@redhat.com>
Acked-by: "Daniel P. Berrangé" <berrange@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240229153017.2221-6-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>

io/channel-file.c

@@ -242,6 +242,11 @@ static int qio_channel_file_close(QIOChannel *ioc,
 {
     QIOChannelFile *fioc = QIO_CHANNEL_FILE(ioc);
 
+    if (qemu_fdatasync(fioc->fd) < 0) {
+        error_setg_errno(errp, errno,
+                         "Unable to synchronize file data with storage device");
+        return -1;
+    }
     if (qemu_close(fioc->fd) < 0) {
         error_setg_errno(errp, errno,
                          "Unable to close file");