virtio: always enter drivers/virtio/

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_vector'
x86_64-linux-ld: eni_vdpa.c:(.text+0x4b7): undefined reference to `vp_legacy_config_vector'
x86_64-linux-ld: drivers/vdpa/alibaba/eni_vdpa.o: in function `eni_vdpa_reset':

When VIRTIO_PCI_LIB was added, it was correctly added to drivers/Makefile
as well, but for the legacy module, this is missing.  Solve this by always
entering drivers/virtio during the build and letting its Makefile take
care of the individual options, rather than having a separate line for
each sub-option.

Fixes: c1ada4b75142 ("vdpa: introduce virtio pci driver")
Fixes: 940e37adecb5 ("eni_vdpa: add vDPA driver for Alibaba ENI")
Fixes: 154349bc5b0d ("virtio-pci: introduce legacy device module")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20211206085034.2836099-1-arnd@kernel.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>