virtio: Revert "virtio: disable multiport console support."

This reverts commit b7a41301.

Multiport support was disabled for 2.6.34 because we wanted to introduce
a new ABI and since we didn't have any released kernel with the older
ABI and were out of the merge window, it didn't make sense keeping the
older ABI around.

Now we revert the patch disabling multiport and rework the ABI in the
following patches.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>