Mark LVM cache pool format as immutable
Mark LVM cache pool format as immutable

The cache pool doesn't have a device node in /dev even when
active and cannot be formatted to anything. By marking the format
immutable we'll make sure that some blivet functions like
recursive_remove will completely ignore it.

vojtechtrefny/blivet,rvykydal/blivet,rvykydal/blivet,vojtechtrefny/blivet