qemu-options: Move -iscsi under "Block device options"

-iscsi ended up under the "Device URL Syntax" heading by a sequence of
errors, as explained in the previous commit.  Move it under the "Block
device options" heading.  Nothing left under "Device URL Syntax";
drop the heading.

Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20171002140307.5292-5-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>