qemu: event: Don't fiddle with disk backing trees without a job

qemu: event: Don't fiddle with disk backing trees without a job

Surprisingly we did not grab a VM job when a block job finished and we'd
happily rewrite the backing chain data. This made it possible to crash
libvirt when queueing two backing chains tightly and other badness.

To fix it, add yet another handler to the helper thread that handles
monitor events that require a job.