Implement degraded status for logical volumes

Logical volumes can be 'degraded' in a similar way to mirrored devices,
when their underlying storage has gone away (i.e. after a physical disk
failure and 'vgreduce --removemissing'). If we can detect this, we can
prevent mistaken replaces of disks that would use this LV (or its
parent) as source data.

This patch adds support for computing the degraded attribute and
modifies gnt-instance to warn if the LV is virtual.

Reviewed-by: imsnah