[master] silence `WARN[0000] Ignoring: vmType vz: [GuestInstallPrefix]`

[AkihiroSuda]

On the current master WARN[0000] Ignoring: vmType vz: [GuestInstallPrefix] is printed for every vz instances

[afbjorklund]

Probably no unit tests for vz, when running on Linux ?

• Allow changing prefix for the installations #1655

There is a hard-coded list, which is Darwin-only:

    if unknown := reflectutil.UnknownNonEmptyFields(l.Yaml, "VMType",