QemuOpts: Convert qemu_opt_set_number() to Error, fix its use

Return the Error object instead of reporting it with
qerror_report_err().

Change callers that assume the function can't fail to pass
&error_abort, so that should the assumption ever break, it'll break
noisily.

Turns out all callers outside its unit test assume that.  We could
drop the Error ** argument, but that would make the interface less
regular, so don't.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
diff --git a/qemu-img.c b/qemu-img.c
index fcdfb67..752eecb 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1550,7 +1550,8 @@
             goto out;
         }
 
-        qemu_opt_set_number(opts, BLOCK_OPT_SIZE, total_sectors * 512);
+        qemu_opt_set_number(opts, BLOCK_OPT_SIZE, total_sectors * 512,
+                            &error_abort);
         ret = add_old_style_options(out_fmt, opts, out_baseimg, NULL);
         if (ret < 0) {
             goto out;