finally, root-on-ffsv2 problem resolved

That took _entirely_ too long and I am glad to not have to think about
this anymore!@#  Oh, and I didn't use sysinst, either--that is not
worth the trouble in this case.  And for the record, I hate grub.

# /dev/rxbd0d:
type: unknown
disk: Xen Virtual ESD
label:
flags:
bytes/sector: 512
sectors/track: 2048
tracks/cylinder: 1
sectors/cylinder: 2048
cylinders: 40960
total sectors: 83886080
rpm: 3600
interleave: 1
trackskew: 0
cylinderskew: 0
headswitch: 0           # microseconds
track-to-track seek: 0  # microseconds
drivedata: 0

16 partitions:
#        size    offset     fstype [fsize bsize cpg/sgs]
 a:  79626240     65536     4.2BSD   2048 16384     0  # (Cyl.     32 -  38911)
 b:   4194304  79691776       swap                     # (Cyl.  38912 -  40959)
 c:  83886080         0     unused      0     0        # (Cyl.      0 -  40959)
 d:  83886080         0     unused      0     0        # (Cyl.      0 -  40959)
 e:     65536         0 Linux Ext2      0     0        # (Cyl.      0 -     31)

apoptosis# dumpfs -s xbd0a
file system: /dev/rxbd0a
format  FFSv2
endian  little-endian
location 65536  (-b 128)
magic   19540119        time    Sun Jun 17 20:00:20 2012
superblock location     65536   id      [ 4fde6354 27c4d26f ]
cylgrp  dynamic inodes  FFSv2   sblock  FFSv2   fslevel 5
nbfree  2360582 ndir    1677    nifree  4812139 nffree  688
ncg     211     size    19906560        blocks  19298838
bsize   16384   shift   14      mask    0xffffc000
fsize   2048    shift   11      mask    0xfffff800
frag    8       shift   3       fsbtodb 2
bpg     11793   fpg     94344   ipg     22912
minfree 5%      optim   time    maxcontig 4     maxbpg  2048
symlinklen 120  contigsumsize 4
maxfilesize 0x000080100202ffff
nindir  2048    inopb   64
avgfilesize 16384       avgfpdir 64
sblkno  40      cblkno  48      iblkno  56      dblkno  2920
sbsize  2048    cgsize  16384
csaddr  2920    cssize  4096
cgrotor 0       fmod    0       ronly   0       clean   0x02
wapbl version 0x1       location 2      flags 0x0
wapbl loc0 39636256     loc1 75360      loc2 512        loc3 7
flags   wapbl
fsmnt   /
volname         swuid   0

I took a hint from the NetBSD/xen howto at
http://wiki.prgmr.com/mediawiki/index.php/NetBSD_as_a_DomU and did
this:

/dev/xbd0a      /       ffs     rw,log  1 1
/dev/xbd0b      none    swap    sw,dp   0 0
/dev/xbd0e      /extboot        ext2fs  ro      1 1