qemu: conf: Enable 'migrate_tls_x509_verify' by default

qemu: conf: Enable 'migrate_tls_x509_verify' by default

The migration stream connection and also the NBD server for non-shared
storage migration don't have any other form of client authentication on
top of the TLS transport, so the only way to authenticate clients is to
verify their certificate.

Enable this option by defauilt when both 'migrate_tls_x509_verify' and
'default_tls_x509_verify' were not configured.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1879477
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>