vhost: Build temporary section list and deref after commit

Igor spotted that there's a race, where a region that's unref'd
in a _del callback might be free'd before the set_mem_table call in
the _commit callback, and thus the vhost might end up using free memory.

Fix this by building a complete temporary sections list, ref'ing every
section (during add and nop) and then unref'ing the whole list right
at the end of commit.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>