If you're concerned about flushing writes consider `mount -oremount,ro` -- this guarantees writes are flushed and is the only truly important step of a system shutdown anyway WRT filesystem integrity. Once filesystems are mounted read-only you can safely power off the machine.
Filesystems can be remounted read-only via the serial console as well, with a break-u. Useful even when userland is otherwise inaccessible such as in the event of a fork bomb.
I usually use `cu`, but the docs for minicom tell me it's control-A followed by F to send a break signal. Then just hit "u." There are a number of single characters following a break with actions to them, the full doc on this kernel feature is here: https://www.kernel.org/doc/Documentation/admin-guide/sysrq.r...
I'm not terribly familiar with KVM but the VM console tools probably have some way to generate a break signal. Check the docs for how to "send a break."
Oh, and don't forget to enable this feature via sysctl -- per the above linux doc.
Filesystems can be remounted read-only via the serial console as well, with a break-u. Useful even when userland is otherwise inaccessible such as in the event of a fork bomb.