frontend: Small changes in spice descriptor generating

1, Adding delete-this-file option in spice descriptor to ensure deletion after
   reading.
2, tls-ciphers row will not be generated when the ciphersuite is null.

Change-Id: I7ed05adcd3fa8c6f93ebe6e77a02710a291f6159
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=965179