Change warning about virtual machine pop-up time of appearance

Hi,

we received a bug report claiming that the pop-up notification of “warning: non-free wirtual machine detected” is disappearing too fast for people to read its full content and then understand why this warning is about.

Related issues

Related to #12003 (closed)
Related to #7439

Original created by @mercedes508 on 16117 (Redmine)

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