init: Add NVME support to the `boot_part_uuid` method of managing boot devices

NVME devices have a subsystem of "class/nvme". Add that to the list of
subsystems we check so we can identify them properly.

Bug: 316324155
Test: Boot on a device with nvme
Change-Id: I2ab3f6890f62f525dba150a0368c666a1c4c875e