qemu-img: Move is_valid_option_list() to qemu-img.c and rewrite

is_valid_option_list()'s purpose is ensuring qemu-img.c's can safely
join multiple parameter strings separated by ',' like this:

        g_strdup_printf("%s,%s", params1, params2);

How it does that is anything but obvious.  A close reading of the code
reveals that it fails exactly when its argument starts with ',' or
ends with an odd number of ','.  Makes sense, actually, because when
the argument starts with ',', a separating ',' preceding it would get
escaped, and when it ends with an odd number of ',', a separating ','
following it would get escaped.

Move it to qemu-img.c and rewrite it the obvious way.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200415074927.19897-9-armbru@redhat.com>