Wait for a couple of minutes until the guest is up and running? I don't know... It seems logical???
Now, if you want to check in your script, you can:
Check for the availability of the guest in the network, either directly or by looking at a shared file in a known network location, or
Loop until you get no error, or
Time with a clock how long it takes your guest to start and put it in a wait timer.
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ERRORLEVEL may one day come up with an error other than 1, so you'd probably want to test:
IF NOT %ERRORLEVEL%==0
Otherwise, a good solution! I have made the host script set a guestproperty to a certain value, and a guest script that runs the guest version of Vboxmanage (VboxControl.exe) to reset the guestproperty. The host script waits until the guestproperty is reset, and proceeds. The guest script is in some startup folder or group policy script, depending on what I needed.