qemu: Reject 'rename-restart' action for 'on_reboot'/'on_poweroff'/'on_crash'

qemu: Reject 'rename-restart' action for 'on_reboot'/'on_poweroff'/'on_crash'

The qemu driver didn't ever implement any meaningful handling for the
'rename-restart' action.

At this point the following handling would take place:

'on_reboot' set to 'rename-restart' is ignored on guest-initiated
reboots, the guest simply reboots.

For on_poweroff set to 'rename-restart' the following happens:

guest initiated shutdown -> 'destroy'
libvirt initiated shutdown -> 'reboot'

In addition when 'on_reboot' is 'destroy' in addition to 'on_poweroff'
being 'rename-restart' the guest is able to execute instructions after
issuing a reset before libvirt terminates it. This will be addressed
separately later.

Forbid the flag in the qemu def validator and update the documentation
to be factual.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>