qemu: conf: Introduce "migrate_tls_force" qemu.conf option

qemu: conf: Introduce "migrate_tls_force" qemu.conf option

Forgetting to use the VIR_MIGRATE_TLS flag with migration can lead to
leak of sensitive information. Add an administrative knob to force use
of the flag.

Note that without VIR_MIGRATE_PEER2PEER, the migration is driven by an
instance of the client library which doesn't necessarily run on either
of the hosts so the flag can't be used to assume VIR_MIGRATE_TLS even
if it wasn't provided by the user instead of rejecting if it's not.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/67
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>