virtio: always enter drivers/virtio/

When neither VIRTIO_PCI_LIB nor VIRTIO are enabled, but the alibaba
vdpa driver is, the kernel runs into a link error because the legacy
virtio module never gets built:

x86_64-linux-ld: drivers/vdpa/alibaba/eni_vdpa.o: in function `eni_vdpa_set_features':
eni_vdpa.c:(.text+0x23f): undefined reference to `vp_legacy_set_features'
x86_64-linux-ld: drivers/vdpa/alibaba/eni_vdpa.o: in function `eni_vdpa_set_vq_state':
eni_vdpa.c:(.text+0x2fe): undefined reference to `vp_legacy_get_queue_enable'
x86_64-linux-ld: drivers/vdpa/alibaba/eni_vdpa.o: in function `eni_vdpa_set_vq_address':
eni_vdpa.c:(.text+0x376): undefined reference to `vp_legacy_set_queue_address'
x86_64-linux-ld: drivers/vdpa/alibaba/eni_vdpa.o: in function `eni_vdpa_set_vq_ready':
eni_vdpa.c:(.text+0x3b4): undefined reference to `vp_legacy_set_queue_address'
x86_64-linux-ld: drivers/vdpa/alibaba/eni_vdpa.o: in function `eni_vdpa_free_irq':
eni_vdpa.c:(.text+0x460): undefined reference to `vp_legacy_queue_vect...