We assume in all the code that a DEVT_DISK uses common/disk.c and/or
common/part.c and we can access a struct disk_devdesc. hostdisk.c
opens raw devices directly, so has no such structures. Define a
kboot-specific DEVT_HOSTDISK and use that instead.
In addition, disk_fmtdev assumes it is working with a struct
disk_devdesc, so write hostdisk_fmtdev as well.
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