Fix error message when TUNNELLED flag is used in non-p2p migration

Fix error message when TUNNELLED flag is used in non-p2p migration

The current error message is

    error: use virDomainMigrateToURI3 for peer-to-peer migration

which is correct but a bit misleading because the client did not specify
VIR_MIGRATE_PEER2PEER flag. This patch changes the error message to

    error: cannot perform tunnelled migration without using peer2peer
    flag

which is consistent with the error reported by older migration APIs.

Reported by Rich Jones in
https://bugzilla.redhat.com/show_bug.cgi?id=1095924