sharing a (mostly) read-only virtual block device

[Antoine Martin <antoine@nagafix.co.uk>]

Hi,

Is there an easy way that I have missed to share a virtual disk
read-only between many guests whilst still having the ability to update
it occasionally from the host?

I was hoping I could use a shared image file, and occasionally replace
it with an updated version (move old copy to disk_image.bak, copy new
image to disk_image), then the guests could umount/mount the drive and
get access to the new disk image.

Unfortunately qemu opens the virtual disk as soon as the guest boots, so
the file descriptor still points to the old image.
Note: I do not want to use the qemu monitor from the host as I want the
guests to be in charge of when/if they get the new disk image.

Any suggestions / other ways of doing this?

Thanks
Antoine