QEMU buildbot

Builds QEMU for Windows.

Notes

• Run on a Debian stretch installation (container is OK)
• The build takes ~1hrs on a single Xeon E5 core (assuming network is good)
• MAKE_FLAGS is by default -j which enables all cores, and this might result in an OOM; set to -j4 or something smaller
• build-stage1.sh must be executed under root; build-stage2.sh can be executed under any user
• SOURCE_BASE_DIR and BUILD_ARTIFACTS_DIR must be writable by the stage2 user; all existing files will be removed in the 2 directories