Because the default wallet has no name, the watch-only and solvables
wallets created during migration end up having no name either.
This fixes it by applying the same prefix name we use for the backup
file for an unnamed default wallet.
Before: watch-only wallet named "_watchonly"
After: watch-only wallet named "default_wallet_watchonly"
Github-Pull: bitcoin/bitcoin#34156
Rebased-From: 82caa8193a