migration: update recv bitmap only on dest vm

We shouldn't update the received bitmap if we're the source VM.  This
fixes a breakage when release-ram is enabled on postcopy.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180723123305.24792-2-peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>