This capability must have the same value on both source and destination,
otherwise migration fails (commit 875fcd01 "migration: incoming
postcopy advise sanity checks").

Let's write it down in various places where postcopy-ram is documented.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <151801810352.29167.4832480228518630626.stgit@bahia.lan>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>