md component detection changes

md component detection changes

Move extra md component detection into the label scan phase.
It had been in set_pv_devices which was deep within the vg_read
phase, which wasn't a good place (better to detect that earlier.)
Now that pv metadata info is available in the scan phase, the pv
details (size and device_hint) can be used for extra md checking.

Use the device_hint from the pv metadata to trigger a full md
component check if the device_hint begins with /dev/md.

Stop triggering full md component checks based on missing
udev info for a dev.

Changes to tests to reflect that the code is now detecting
md components in some test case that it wasn't before.