When a VM (or its template) does not explicitly set a qrexec_timeout,
fall back to a global default_qrexec_timeout (with default value 60),
instead of hardcoding the fallback value to 60.
This makes it easy to set a higher timeout for the whole system, which
helps users who habitually launch applications from several (not yet
started) VMs at the same time. 60 seconds can be too short for that.
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