Fix gparted scans forever blank disk in virtual machine (#697518)

In some circumstances gparted would appear to scan forever if it
encountered a blank hard disk.  This would happen if the timing of
events was right and gparted encountered a disk without a partition
table.  The missing partition table would cause a call to the
exception handler, which could get stuck in a thread waiting position.

The problem was traced back to the ped_exception_handler
and a cond.signal() call missing the requisite mutex.lock() and
mutex.unlock() around the signal call.

Closes Bug #697518 - gparted scans forever blank disk in virtualbox