Code review comment for ~ltrager/maas:lp1813019

VMFS6 requires at least 10G of space. In the VMFS6 review I upped LARGE_BLOCK_DEVICE to 100GB in maasserver.tests.test_storage_layouts to ensure unit tests pass. sampledata uses factory.make_PhysicalBlockDevice which allows a disk to be created as small as 64MB. This caused intermittent fails such as 5491 as well as cause make sampledata to fail.