During install tests, avoid VM being shutdown before PDB can run

[ydirson]

When booting a nested host with an installer ISO, in case of failure to get to the state where we can read it using ssh, we could not inspect the host once --pdb stopped the test, because the exception was firing from fixture setup code.

[glehmann]

As discussed earlier, it would be nice if we could avoid yielding None :)

[stormi]

As discussed earlier, it would be nice if we could avoid yielding None :)

Is it a request for change, or just a suggestion but then you didn't approve for some reason?

[vxgmichel]

I think this change is very useful, but I agree with @glehmann that it might done in a way that is more straight forward.

In particular, having (try_)booting_vm_installer no longer failing when it could not boot gives the wrong impression that this might be expected for some specific tests. As far as I understand this is not the case, but a way to address the valid issue of being able to drop in a PDB before the cleanup is performed when the booting fails.

Instead, we might want to add a defer/request.addFinalizer to booting_vm_installer, i.e remove the try-except-finally statements and use something like this instead:

    def cleanup():
        if host_vm is not None:
            host_vm.shutdown(force=True)
        if remote_iso:
            host.pool.remove_iso(remote_iso)
    defer(cleanup)

This should achieve the expected result.