cros_vm_lib: Fallback VM baseline to Sandybridge

As current lab still use baremetal with Sandybridge, downgrade it to
Sandybridge.

BUG=chromium:808045
TEST=None.
CQ-DEPEND=CL:905993
CQ-DEPEND=CL:906681

Change-Id: Ia548ee776a3d5c8cf4f4bd301c9d14b228706f8e
Reviewed-on: https://chromium-review.googlesource.com/905990
Commit-Ready: Ilja H. Friedel <ihf@chromium.org>
Tested-by: Ilja H. Friedel <ihf@chromium.org>
Reviewed-by: Ilja H. Friedel <ihf@chromium.org>
1 file changed