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 ~3.5.

v3:
 - Including a memset optimization.
v2:
 - Resend as non-RFC.

==
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-v3:
https://github.com/jnsnow/qemu/releases/tag/qtest_protocol-v3
==