vm_control: minor style improvements for powerbtn command
vm_control: minor style improvements for powerbtn command

Minor style improvement - no functional changes.

BUG=None
TEST=Make sure that "crosvm powerbtn /run/<>/crosvm.sock" works as
expected.

Change-Id: Ide843707d69c25c9e0473bc8e55db8c4a6da7553
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3616883
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Commit-Queue: Grzegorz Jaszczyk <c5b88449c8a12e1c1d01de50f0c2b638f2b76073@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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