iotests: Remove raciness from 162

With qemu-nbd's new --fork option, we no longer need to launch it the
hacky way.

Suggested-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
