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This CL fixes a crash that can occur when a USB device shared with a VM
is shared with a different VM. If the device has not yet been attached,
e.g. as the first VM is not yet started, we end up in a recursive loop
between CrosUsbDetector::AttachUsbDeviceToVm(), DetachUsbDeviceFromVm()
and AttachAfterDetach(), eventually getting a stack overflow.

Bug: b/171265431
Change-Id: I0ecc28e55160c310473dce3a98f8ada0f99eb3a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485660
Commit-Queue: Timothy Loh <timloh@chromium.org>
Commit-Queue: Nicholas Verne <nverne@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819233}