crosvm: add missing doc comments for configure_vcpu method.
crosvm: add missing doc comments for configure_vcpu method.

BUG=None
TEST=Run "cargo doc" generate documents and check the arguments
description is correct.

Change-Id: I481d950928587728fdb69215b867561d6752f4aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3670720
Commit-Queue: Junichi Uekawa <6bd240a6ce32463e272bb69c685a8e946b33ac05@chromium.org>
Reviewed-by: Junichi Uekawa <6bd240a6ce32463e272bb69c685a8e946b33ac05@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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