RSC_VIRTIO_CFG isn't being used, so remove it.

Originally it was introduced to overcome a resource table limitation
that prevented describing a virtio device in a single resource table
entry.

The plan though is to describe resource table entries in a TLV fashion,
where each entry will consume the amount of space it requires,
so the original limitation is anyway temporary.
Reported-by: NStephen Boyd <sboyd@codeaurora.org>
Signed-off-by: NOhad Ben-Cohen <ohad@wizery.com>