qemu-img: Amendment support implies create_opts

Instead of checking whether a driver has a non-NULL create_opts we
should check whether it supports image amendment in the first place.  If
it does, it must have create_opts.

On the other hand, if it does not have create_opts (so it does not
support amendment either), the error message "does not support any
options" is a bit useless.  Stating clearly that the driver has no
amendment support whatsoever is probably better.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-id: 20180509210023.20283-2-mreitz@redhat.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

qemu-img.c

@@ -3789,13 +3789,16 @@ static int img_amend(int argc, char **argv)
         goto out;
     }
 
-    if (!bs->drv->create_opts) {
-        error_report("Format driver '%s' does not support any options to amend",
+    if (!bs->drv->bdrv_amend_options) {
+        error_report("Format driver '%s' does not support option amendment",
                      fmt);
         ret = -1;
         goto out;
     }
 
+    /* Every driver supporting amendment must have create_opts */
+    assert(bs->drv->create_opts);
+
     create_opts = qemu_opts_append(create_opts, bs->drv->create_opts);
     opts = qemu_opts_create(create_opts, NULL, 0, &error_abort);
     qemu_opts_do_parse(opts, options, NULL, &err);