[moblab] Include a retry when searching for the USB drive.

the moblab vm has a flake where it fails to find the USB drive.
As the image to provision is installed on the USB drive when the
VM is created it is important to have this work.

From reading lsusb -v will force a rescan of the USB devices.

TEST=local moblab, tryjobs
BUG=chromium:929438

Change-Id: Ia09a9b647c6bc8631a734c9e7d6353c1c4ff038f
Reviewed-on: https://chromium-review.googlesource.com/1477984
Commit-Ready: Keith Haddow <haddowk@chromium.org>
Tested-by: Keith Haddow <haddowk@chromium.org>
Reviewed-by: Matt Mallett <mattmallett@chromium.org>
1 file changed