gpt-auto-generator: enable auto-discovery logic also for verity root file systems

gpt-auto-generator: enable auto-discovery logic also for verity root file systems

verity block devices have two backing devices: the data partition and
the hash partition. Previously the gpt auto-discovery logic would refuse
working on devices with multiple backing devices, losen this up a bit,
to permit them as long as the backing devices are all located on the
same physical media.