Adds new qtest protocol commands for base64 reads and writes,
as well as a proper command for memset instead of faking it
via write.

This improves the ahci-test performance on my machine from about
14 seconds to about 4.

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

This version is tagged qtest_protocol-v2:
https://github.com/jnsnow/qemu/releases/tag/qtest_protocol-v2
==