qemu-doc: Remove paragraph about requiring a HD image with -kernel

qemu-doc: Remove paragraph about requiring a HD image with -kernel

The need for specifying "-hda" together with "-kernel" has been removed in
commit 57a46d057995 ("Convert linux bootrom to external rom and fw_cfg"),
almost 10 years ago, so let's remove this description from our documentation
now, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20191001110111.4870-1-thuth@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>