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Previously if QEMU ran but neither exited prematurely, nor did the
guest ever become connectible via SSH, then the QEMU sub-process would
be left running after the runner script exited.

Bug: 877194
Change-Id: Id50fa44e11987f69afc469d73adc445b5066b7a5
Reviewed-on: https://chromium-review.googlesource.com/1187543
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585709}