sys_util: simplify format string for Display impl for Priority
sys_util: simplify format string for Display impl for Priority

Change-Id: I2c08c707b0e5c8d576aeade94265434f126c6927
Reviewed-on: https://chromium-review.googlesource.com/1277870
Commit-Ready: ChromeOS CL Exonerator Bot <fa429e5907f00867f797423aea74327bb02c91b6@appspot.gserviceaccount.com>
Tested-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

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