Code review comment for ~mwhudson/curtin:lp-1893818

So the new vmtests passed. Of the old multipath tests, Centos70TestMultipathBasic fails but it also fails the same way on master (like this:

[ 214.243025] cloud-init[1257]: start: cmd-install/stage-late/00_grub1_boot: running 'curtin in-target -- sed -i.curtin -e s|(hd1,0)|(hd0,0)|g /boot/grub/grub.conf'
[ 214.710207] cloud-init[1257]: start: cmd-install/stage-late/00_grub1_boot/cmd-in-target: curtin command in-target
[ 214.718102] cloud-init[1257]: Running command ['mount', '--bind', '/dev', '/tmp/tmp69un0w5l/target/dev'] with allowed return codes [0] (capture=False)
[ 214.726514] cloud-init[1257]: Running command ['mount', '--bind', '/proc', '/tmp/tmp69un0w5l/target/proc'] with allowed return codes [0] (capture=False)
[ 214.734124] cloud-init[1257]: Running command ['mount', '--bind', '/run', '/tmp/tmp69un0w5l/target/run'] with allowed return codes [0] (capture=False)
[ 214.741353] cloud-init[1257]: Running command ['mount', '--bind', '/sys', '/tmp/tmp69un0w5l/target/sys'] with allowed return codes [0] (capture=False)
[ 214.749061] cloud-init[1257]: Running command ['unshare', '--help'] with allowed return codes [0] (capture=True)
[ 214.755668] cloud-init[1257]: Running command ['unshare', '--fork', '--pid', '--', 'chroot', '/tmp/tmp69un0w5l/target', 'sed', '-i.curtin', '-e', 's|(hd1,0)|(hd0,0)|g', '/boot/grub/grub.conf'] with allowed return codes [0] (capture=False)
[ 214.760850] cloud-init[1257]: sed: can't read /boot/grub/grub.conf: No such file or directory
[ 214.764817] cloud-init[1257]: Running command ['udevadm', 'settle'] with allowed return codes [0] (capture=False)
[ 214.784140] cloud-init[1257]: TIMED subp(['udevadm', 'settle']): 0.018
[ 214.785396] cloud-init[1257]: Running command ['umount', '/tmp/tmp69un0w5l/target/sys'] with allowed return codes [0] (capture=False)
[ 214.792526] cloud-init[1257]: Running command ['umount', '/tmp/tmp69un0w5l/target/run'] with allowed return codes [0] (capture=False)
[ 214.799577] cloud-init[1257]: Running command ['umount', '/tmp/tmp69un0w5l/target/proc'] with allowed return codes [0] (capture=False)
[ 214.806709] cloud-init[1257]: Running command ['umount', '/tmp/tmp69un0w5l/target/dev'] with allowed return codes [0] (capture=False)
[ 214.813171] cloud-init[1257]: finish: cmd-install/stage-late/00_grub1_boot/cmd-in-target: FAIL: curtin command in-target
[ 214.858039] cloud-init[1257]: 00_grub1_boot command failed

)