Revert "vhost: access vq metadata through kernel virtual address"

This reverts commit 7f466032 ("vhost: access vq metadata through
kernel virtual address").  The commit caused a bunch of issues, and
while commit 73f628ec ("vhost: disable metadata prefetch
optimization") disabled the optimization it's not nice to keep lots of
dead code around.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>