conf: Validate firmware configuration more thoroughly

conf: Validate firmware configuration more thoroughly

Generally speaking, when firmware autoselection is in use we
don't want any information to be provided manually. There are
two exceptions:

  * we still want the path to the NVRAM file to be customizable;

  * using <loader secure='yes'/> was how you would ask for a
    firmware that implements the Secure Boot feature in the
    original approach to firmware autoselection, so we want to
    keep that working.

Anything else should result in a descriptive error.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/327
Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>