qom: strdup() target property name on object_property_add_alias()

With this, object_property_add_alias() callers can safely free the
target property name, like what already happens with the 'name' argument
to all object_property_add*() functions.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>