mkosi: fix bootable + verity /usr/ builds
mkosi: fix bootable + verity /usr/ builds

This fixes two issues with UsrOnly=yes builds that are bootable and have
verity:

1. use the right partition type uuid for the verity partition
2. fix the umount path when we invoke dracut in a /usr-only build, since
   "umount --recursive" insists on having a mount point at the specified
   path, which we previously didn't necessarily. Let's add a bind mount
   on itself there to address this.

A follow-up for: #676

systemd/mkosi,systemd/mkosi