Libvirt: enable error_policy option for instances

By default, when a libvirt VM encounters a disk error reported from the underlying disk subsystem, it reports this error through to the guest OS in the VM. As a result of this the guest OS usually encounters an error and enters a bad state. This could be a crash, or it could be the file systems inside the guest VM going into read only.

libvirt provides an option, 'error_policy', to configure how to handle these types of errors. We should be able to configure the error policy to do other actions as desired by users. The most obviously useful is error_policy="stop", which makes the VM go into a paused state. In some cases, it may be preferable to pause VMs with I/O errors until the error condition can be resolved, rather than sending them all into an error state.