Why is xyzal (levocetirizine) given to a patient with systemic viral infection?..it is because of a possibility of having an allergy caused by the virus itself?

Not sure: Usually xyzal, (levocetirizine) or levoceterizine, is reserved for allergic problems. It it not typically used for uri's (colds). But some people have rashes/hives associated with uris and this can be helpful. Sometimes it is recommended for the nasal symptoms, but efficacy can be variable. But yes, you can have allergic reactions triggered by infections.