drives: Fix typo in error message.

This fixes commit 0a065c45748ec5efe86f4d02f4864a58ed7dae36.
iscsi: Document that one server must be passed, and that username may be passed.

This fixes commit 8b271101a1724bab926c65c77436d8128bf1c2df.
Add support for qemu's curl driver (ie. FTP, FTPS, HTTP, HTTPS, TFTP).