target-i386: Prepare "vendor" property for X86CPU

Using it now would incur converting the three x86_def_t vendor words
into a string for object_property_set_str(), then back to three words
in the "vendor" setter.
The built-in CPU definitions use numeric preprocessor defines to
initialize the three words in a charset-safe way, so do not change the
fields to char[12] just to use the setter.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
