qemu: conf: Enable 'backup_tls_x509_verify' by default

qemu: conf: Enable 'backup_tls_x509_verify' by default

The NBD server used to export pull-mode backups doesn'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 'backup_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>