cvtnum returns an int64_t, not an int, so correct the lvalue types
wherever it is used. While we're at it, make the error messages more
meaningful and hopefully less confusing.

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For convenience, this branch is available at:
https://github.com/jnsnow/qemu.git branch qemu-io-tidy
https://github.com/jnsnow/qemu/tree/qemu-io-tidy

This version is tagged qemu-io-tidy-v1:
https://github.com/jnsnow/qemu/releases/tag/qemu-io-tidy-v1