virLXCProcessReboot: Remove the need to re-register autodestroy callback

virLXCProcessReboot: Remove the need to re-register autodestroy callback

Add a new flag VIR_LXC_PROCESS_CLEANUP_AUTODESTROY to
virLXCProcessCleanupFlags for skipping removal of the autodestroy
callback so that fake reboot of the container doesn't need to fetch the
connection and re-register it.

Since virLXCProcessReboot is defined before virLXCProcessCleanupFlags,
this patch also moves the flag enum typedef to the beginning of the
file.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Tim Wiederhake <twiederh@redhat.com>