Support RAID devices

## Problem
Unable to install on a RAID device (e.g. RAID0 striped as some laptop manufacturers decided to put in 🙄). The installer currently shows the underlying physical disks and not any of the existing partitions on top of the RAID layer.

Physical disks:
/dev/sda (128GB)
/dev/sdb (128GB)

RAID0 layer:
/dev/dm-0 (256GB)

GPT partition table with:

Number Start (sector) End (sector) Size Code Name
1 2048 1023999 499.0 MiB 2700 Basic data partition
2 1024000 1228799 100.0 MiB EF00 EFI system partition
3 1228800 1261567 16.0 MiB 0C01 Microsoft reserved ...
4 1261568 90519551 42.6 GiB 0700 Basic data partition
5 90519552 500121566 195.3 GiB 8E00 Linux LVM

## Proposal
I suspect it may all *just work* if there was a way to make the installer ignore the underling devices and treat the /dev/dm-0 as the physical disk.

Alternatively if it's a bit too niche for what you're wanting to support I'd love any suggestions for workarounds, I don't mind doing a more manual install if I can get there in the end!

## Prior Art
In the default Ubuntu installer which was used previously it was always possible to install Elementary to the right place (though the RAID definitely made it a bit confusing.