devices:vfio: Remove vfio group when vfio device is closed
devices:vfio: Remove vfio group when vfio device is closed

One container could contains multi groups, one group could contains
multi devices, when all the devices in the group are closed, the
group itself should be removed also.

BUG=b:185084350
TEST=Boot a guest with and without passthrough device

Change-Id: Ia43da3e2e9ad58d0a7596141645926931c0bcc5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2955572
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm