Request erroring out from the backing chain traveller and drop qemu's
internal backing chain integrity tester.

The backing chain traveller reports errors by itself with possibly more
detail than qemuDiskChainCheckBroken ever could.

We also need to make sure that we reconnect to existing qemu instances
even at the cost of losing the backing chain info (this really should be
stored in the XML rather than reloaded from disk, but that needs some
work).