Make DRBD8 disks show 'degraded' status if diskless

This patch enables the bdev.DRBD8 class report a degraded status if the
local disk is missing. This allows `gnt-instance info` to report the
actual situation in this case.

Note that DRBD7 should also behave like this, however the diskless case
is less often met there and we also don't want to change behaviour.

The patch also fixes some docstrings for the GetSyncStatus methods.

Reviewed-by: imsnah