this wiki page explains it and it can be done via GUI.
option 'startup'Is there a way to set boot order / priority via command line / qm?
--startup `[[order=]\d+] [,up=\d+] [,down=\d+] `
Startup and shutdown behavior. Order is a non-negative number defining the general startup order. Shutdown in done with reverse ordering. Additionally you can set the up or down delay in seconds, which specifies a delay to wait before the next VM is started or stopped.
option 'startup'
is a little bit confusing.--startup `[[order=]\d+] [,up=\d+] [,down=\d+] `
?--startup order=<integer>,up=<integer>,down=<integer>