bionic install with D-I produces corrupt filesystem

[Doug Zongker]

Repro:
1) install to empty disk image from ubuntu-18.04-server-amd64.iso
2) accept all defaults except don't use LVM (not sure if this makes a difference, but I pick "Guided - use entire disk")
3) run e2fsck on resulting disk image, see errors

I think this happens because the target disk is not correctly unmounted prior to being powered off:

May 17 22:57:39 finish-install: info: Running /usr/lib/finish-install.d/65partman-md
May 17 22:57:39 finish-install: info: Running /usr/lib/finish-install.d/70mtab
May 17 22:57:39 finish-install: info: Running /usr/lib/finish-install.d/90base-installer
May 17 22:57:39 finish-install: info: Running /usr/lib/finish-install.d/90console
May 17 22:57:39 finish-install: info: Running /usr/lib/finish-install.d/94random-seed
May 17 22:57:39 finish-install: info: Running /usr/lib/finish-install.d/94save-logs
May 17 22:57:40 finish-install: info: Running /usr/lib/finish-install.d/95umount
May 17 22:57:40 finish-install: umount: can't unmount /target: Device or resource busy
May 17 22:57:40 finish-install: umount: can't unmount /dev/pts: Device or resource busy
May 17 22:57:40 finish-install: umount: can't unmount /dev: Device or resource busy
May 17 22:57:40 finish-install: umount: can't unmount /run: Device or resource busy
May 17 22:57:40 finish-install: umount: can't unmount /: Invalid argument
May 17 22:57:40 finish-install: info: Running /usr/lib/finish-install.d/97release-dhcp-lease
May 17 22:57:40 finish-install: info: Running /usr/lib/finish-install.d/98exit-installer
May 17 22:57:40 finish-install: warning: /usr/lib/finish-install.d/98exit-installer returned error code 1
May 17 22:57:40 finish-install: info: Running /usr/lib/finish-install.d/99reboot