Now that we have @SysEmuTarget, it makes sense to restrict
@TargetInfo.@arch to valid sysemu targets at the schema level.

Cc: "Daniel P. Berrange" <berrange@redhat.com>
Cc: Eric Blake <eblake@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180427192852.15013-4-lersek@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>