Currently, all of the VirtioOptions are under a single <optional>
element, however, neither our parser/formatter or QEMU driver requires
the presence of all the options if only a single one from the set has
been specified, so fix it and silence the schema validator.
Signed-off-by: NErik Skultety <eskultet@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>