qemu: fill out usage-specific TLS settings after parsing

qemu: fill out usage-specific TLS settings after parsing

Instead of copying the default default values upfront
and then wondering whether the user has given us a new default,
leave the per-usage TLS certdirs and secrets empty during
parsing and only fill them afterwards if they weren't provided
by the user.

This means that instead of looking whether the specific certdir
paths match the default default, the Validate function (which
is called in between parsing and setting the defaults) can error
out for missing directories if the value is present, because
it must've come from the user.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>