conf: Reject enrolled-keys=yes with secure-boot=no

conf: Reject enrolled-keys=yes with secure-boot=no

This combination doesn't make sense and so the firmware
autoselection logic will not be able to find a suitable firmware,
but it's more user-friendly to report a detailed error upfront.

Note that this check would ideally happen in the validate phase,
but if we moved it there we would no longer be able to
automatically enable secure-boot when enrolled-keys=yes. Since
the combination never resulted in a working configuration, the
chances of this causing real-world VMs to disappear are
extremely low.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>