can't use swap partition from dos ptable

[Olivier Gayot]

Steps to reproduce:

1. Boot the server installer (scripts/kvm-test.py works fine)
2. Use fdisk to create the following disk layout:
  Disklabel type: dos

  Device Boot Start End Sectors Size Id Type
  /dev/vdb1 2048 2099199 2097152 1G 82 Linux swap / Solaris
  /dev/vdb2 2099200 23070719 20971520 10G 83 Linux
  /dev/vdb3 23070720 25165823 2095104 1023M ef EFI (FAT-12/16/32)

3. Navigate to the partitioning screen and select custom storage layout
4. Edit partition #1, change format to swap
5. Edit partition #2, change format to ext4. Ensure it gets mounted at /
6. Validate the changes and pursue the installation.
7. curtin partitioning will fail with:

  RuntimeError: Verifying /dev/vdb1 partition flag, expecting swap, found None

[Olivier Gayot (ogayot)]

Also happens when using storage-version 2 (e.g., on the desktop installer)