vm factory may return vm=nil, err=nil for GetVm

[gnawux]

This is the root cause of hyperhq/hyperd#612 . The case is covered by hyperhq/hyperd#615, however, it should not return err=nil in a failed case.

[laijs]

it can't be found after careful review...

[gnawux]

fixed by #504

[laijs]

#504 is unrelated to this problem.

[gnawux]

but looks really like this case