virt-host-validate: Call VIR_HOST_VALIDATE_FAILURE() more frequently

virt-host-validate: Call VIR_HOST_VALIDATE_FAILURE() more frequently

Ideally, every virHostMsgFail() would be coupled with
VIR_HOST_VALIDATE_FAILURE() so that the failure is correctly
propagated to the caller. However, in
virHostValidateSecureGuests() we are either ignoring @level and
returning 0 directly (no error), or not returning at all, relying
on 'return 0' at the end of the function. Neither of these help
propagate failure correctly.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Fabiano Fidêncio <fabiano@fidencio.org>