If a HV parameter is required that does not apply for an instance,
currently the code returns None. This is bad, as it means we cannot
switch to the actual HV parameter types and validate correctly this
field.

This patch changes it so that in this case we return QRFS_UNAVAIL;
ideally we would use a NOT_APPROPRIATE or similar field, but UNAVAIL is
good enough (the call cannot fail in another way).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>