Bear in mind plenty of viruses do not have any DNA, using RNA instead. But there are other Cas proteins that cut RNA, so you can still apply that kind of approach.

It's not necessarily going to be all that effective for typical viral infections, as it's hard to deliver a lot of CRISPR-Cas machinery in vivo, whereas a viral infection can create huge numbers of viruses.

Where it could be exciting is in potentially permanently curing HIV infection. You use other drugs to knock the infection down, but some of the viruses have integrated into the DNA of host cells, where drugs do no good. But CRISPR-Cas9 could come along and destroy those viral DNA sequences.