Do not probe dmverity and removable devices

Dear Maintainers,

This MR does not probe partitions that belong to a verity device-mapper (first commit) and to a removable media (second commit).

Verity needs two devices (data and hash) and creates a device-mapper device; the last one is usually mounted but not the other. Therefore, os-prober lists the data device as an OS even if it is the root device because it is not mounted. The first commit lists the two devices (data and hash) like it is done already for the RAID devices.

I am not sure about how the second commit is breaking the existing world. I would like os-prober to not list the OS from the ISO that was just dd'ed to the removable media when I upgrade the GRUB configuration file; this creates a non-relevant entry in the menu as the media is not necessarily plugged to the machine.