vl: Clean up -smp error handling

The Error ** argument must be NULL, &error_abort, &error_fatal, or a
pointer to a variable containing NULL.  Passing an argument of the
latter kind twice without clearing it in between is wrong: if the
first call sets an error, it no longer points to NULL for the second
call.

machine_parse_property_opt() is wrong that way: it passes @errp to
keyval_parse() without checking for failure, then passes it to
keyval_merge().  Harmless, since the only caller passes &error_fatal.

Clean up: drop the parameter, and use &error_fatal directly.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210720125408.387910-16-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
[Rebased, conflict with commit a3c2f128 resolved]