[ZSvirt] bug: sdk: error message references non-existent setConfig() method

[chayu163]

Description

In org.zstack.sdk.ZSClient.errorIfNotConfigured, when the client is used
before being configured, an error is thrown with:

throw new RuntimeException("setConfig() must be called before any methods");

However, ZSClient exposes configure(ZSConfig c) as its public
initialization method — there is no setConfig() method. The error message
points users to a method that does not exist, which is misleading when
troubleshooting this failure.

Expected behavior

The error message should name the actual method to call.

Proposed fix

throw new RuntimeException("configure() must be called before any methods");