Re: [Libguestfs] [PATCH] v2v: Give better diagnostics if filesystem falls back to read-only (RHBZ#1567763).

Some filesystems fall back silently to read-only if there are problems such a dirty filesystem and an unrecoverable journal. Almost all conversions involve writing to the root filesystem, so these will inevitably fail later on with a strange error message. Test the root filesystem is writable by creating and deleting a temporary file, and if the creation fails then give better diagnostics.