stand/kboot: hostdisk isn't a DEVT_DISK, use a different value.

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.

Sponsored by: Netflix

(cherry picked from commit 2cb90a7b2efc41e791c589e17127f63ccf24167c)