New pmboostrap required host programs not found on Debian

Since #2450, the new requirements are not found on Debian, as losetup and kpartx are root only commands (under /usr/sbin), as such they are not found by shutil.which().

This is the output of which for those two commands on my system:

$ which losetup
$ sudo which losetup
/usr/sbin/losetup
$ which kpartx
$ sudo which kpartx
/usr/sbin/kpartx

I don't know what the correct fix would be.