qemu: validate: Allow 'preserve' action for on_crash lifecycle action

qemu: validate: Allow 'preserve' action for on_crash lifecycle action

In fact keeping the VM around for debugging is a desirable configuration
and actually the implementation has no code as we keep the VM around.

Remove the validation and add a note that it's actually used.

Fixes: b1b85a475fb251b9068b75f629479f5c452f1b43
Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.ibm.com>