Additional check for qemu-nbd hang

/sys/block/*device*/pid check is not enough. I see that the unix
socket used by the device my be stuck as well, so let's add another
check for the path to the unix socket for the device as well to
figure out if the device is free. Complain loud and clear that the
qemu-nbd is leaking resources.

Change-Id: I28cedffba7a9915ef6f7888989e40e4d0de475c6
Closes-Bug: #973413
Partial-Bug: #1254890
Merge "Additional check for qemu-nbd hang"