vhost+postcopy: Send address back to qemu

We need a better way, but at the moment we need the address of the
mappings sent back to qemu so it can interpret the messages on the
userfaultfd it reads.

This is done as a 3 stage set:
   QEMU -> client
      set_mem_table

   mmap stuff, get addresses

   client -> qemu
       here are the addresses

   qemu -> client
       OK - now you can use them

That ensures that qemu has registered the new addresses in it's
userfault code before the client starts accessing them.

Note: We don't ask for the default 'ack' reply since we've got our own.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>