Additional stats for podman info

In support of podman machine and its counterpart desktop, we have added
new stats to podman info.

For storage, we have added GraphRootAllocated and GraphRootUsed in
bytes.

For CPUs, we have added user, system, and idle percents based on
/proc/stat.

Fixes: #13876

Signed-off-by: Brent Baude <bbaude@redhat.com>

docs/source/markdown/podman-info.1.md

@@ -39,6 +39,10 @@ host:
     package: conmon-2.0.29-2.fc34.x86_64
     path: /usr/bin/conmon
     version: 'conmon version 2.0.29, commit: '
+ cpu_utilization:
+   idle_percent: 96.84
+   system_percent: 0.71
+   user_percent: 2.45
   cpus: 8
   distribution:
     distribution: fedora
@@ -124,6 +128,8 @@ store:
   graphDriverName: overlay
   graphOptions: {}
   graphRoot: /home/dwalsh/.local/share/containers/storage
+  graphRootAllocated: 510389125120
+  graphRootUsed: 129170714624
   graphStatus:
     Backing Filesystem: extfs
     Native Overlay Diff: "true"