fix #29056122

commit 'fbb2f06e' ("pvpanic: add crash loaded event") introduce new
pvpanic event 'PVPANIC_CRASH_LOADED', it make the qemu on host can get
info that if the guest already handle the panic by kdump or not.

But if the guest enabled the kdump, it will not post the panic event by
default unless the parameter 'crash_kexec_post_notifiers' is given.
So, its better to set the default value of this parameter to true, to
avoid it missed in case of kdump enabled.

If user want disable the event notification, the parameter
'crash_kexec_post_notifiers=N' should be given.
Signed-off-by: NShile Zhang <shile.zhang@linux.alibaba.com>
Acked-by: NJoseph Qi <joseph.qi@linux.alibaba.com>