vhost: revert "vhost: disable for OABI"

This reverts commit d085eb8c

 ("vhost: disable for OABI")
With commit "virtio: force spec specified alignment on types"
in place, we force proper alignment for all structures,
so there's no longer a reason to blacklist OABI.

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