Fix detection of busy status of extended partitions (#712533)

GParted doesn't report extended partitions as busy if it contains only
busy LVM2 PV or SWRAID logical partitions.  Libparted's
ped_partition_is_busy() only detects mounted file systems and swap space
as busy, not active LVM2 PVs or SWRAID members.  This is as of libparted
3.1 and earlier.

Fix by determining the busy status of an extended partition based solely
on the busy status of the logical partitions it contains.  This makes it
unnecessary to check for mounted DMRAID logical partitions or call
ped_partition_is_busy() for extended partitions.

Bug #712533 - Partitions not detected as busy inside Software RAID on
              some distros