scripts: Fix FLAGS_moblab usage.

Define the usb_devices and moblab flags in the lib/cros_vm_lib
script rather than in bin/cros_start_vm as they are only used
with lib/cros_vm_lib.

BUG=chromium:550695
TEST=Tested running the script.

Change-Id: I3715cafe9b4c47db5b7d9e72edee723745b25417
Reviewed-on: https://chromium-review.googlesource.com/310282
Commit-Ready: Matthew Sartori <msartori@chromium.org>
Tested-by: Matthew Sartori <msartori@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
2 files changed