If a VM disk points a physical block device (‘phy’) that is not present at system boot, xenmgr will exit with a message such as “Caught error: Bad Physical Path for disk 3 (/dev/foo). Stopping RPC server”, making the system unusable and unresponsive to further commands, including xec-vm. The only workaround is to edit the VM config file to remove the disk definitions, restart dbd, then start xenmgr.
Instead, xenmgr should print a warning message and then ignore the VM whose disk cannot be found. That disk could be an external USB storage device to be attached later. There are potential use cases where hot plug of a storage device could cause a VM to be launched. In that scenario, the VM physical path would be defined as /dev/disk/by-id/foo-guid, and the symlink would not be present until the USB device is hot plugged.