fix hardcoded CT uptime in vmstatus

fix hardcoded CT uptime in vmstatus

Implement the container uptime by susbtracting the ctime from the
container pid file from the actual time.
This mirrors the behaviour of lxcfs, see get_pid1_time() in lxcfs.c.
This hass some limitations, like frozen or live migrated containers
falsify the real uptime. But as it shows everytime the uptime like
a uptime command in the container would this is forgivable, for now.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>