bhyve: fix disks address allocation
    
    As bhyve currently doesn't use controller addressing and simply
    uses 1 implicit controller for 1 disk device, the scheme looks the
    following:
    
     pci addrees -> (implicit controller) -> disk device
    
    So in fact we identify disk devices by pci address of implicit
    controller and just pass it this way to bhyve in a form:
    
     -s pci_addr,ahci-(cd|hd),/path/to/disk
    
    Therefore, we cannot use virDeviceInfoPCIAddressWanted() because it
    does not expect that disk devices might need PCI address assignment.
    
    As a result, if a disk was specified without address, it will not be
    generated and domain will to start.
    
    Until proper controller addressing is implemented in the bhyve
    driver, force each disk to have PCI address generated if it was not
    specified by user.