PCI device enumeration is one of a large number of concepts that transitioned from the physical world. Originally, being a bus and a slot number now, of course, is a virtualised concept.

When OS boots during the POST process the local devices are enumerated (checked for presence and size). If an expected device is found at the PCI slot that is a failure of the test. 

In AHV deployments, the Controller VM (CVM) runs as a VM and disks are presented using PCI passthrough. That means that if AHV PCI devices enumeration has changed CVM may not be aware of the fact and attempt to direct I/O to the devices that are no longer accessible via known PCI bus and slot. 

This can happen after hardware replacement on an AHV host and the CVM will not boot or will boot with only part of the expected devices accessible. 

Compare PCI slot numbers of SCSI controllers on AHV host and the CVM and update the enumeration of the devices on the CVM. 

For more details look at KB-7154 AHV | CVM might not boot after hardware replacement due to PCI re-numbering