qemu-img: Recognize no creation support in -o help

qemu-img: Recognize no creation support in -o help

The only users of print_block_option_help() are qemu-img create and
qemu-img convert for the output image, so this function is always used
for image creation (it used to be used for amendment also, but that is
no longer the case).

So if image creation is not supported by either the format or the
protocol, there is no need to print any option description, because the
user cannot create an image like this anyway.

This also fixes an assertion failure:

    $ qemu-img create -f bochs -o help
    Supported options:
    qemu-img: util/qemu-option.c:219:
    qemu_opts_print_help: Assertion `list' failed.
    [1]    24831 abort (core dumped)  qemu-img create -f bochs -o help

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20180509210023.20283-6-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>