iotests: Remove raciness from 162

With qemu-nbd's new --fork option, we no longer need to launch it the
hacky way.
Suggested-by: NSascha Silbe <silbe@linux.vnet.ibm.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/162

@@ -51,8 +51,7 @@ $QEMU_IMG info 'json:{"driver": "nbd", "host": "does.not.exist.example.com", "po
 # This is a test for NBD's bdrv_refresh_filename() implementation: It expects
 # either host or path to be set, but it must not assume that they are set to
 # strings in the options QDict
-$QEMU_NBD -k "$PWD/42" -f raw null-co:// &
-sleep 0.5
+$QEMU_NBD -k "$PWD/42" -f raw --fork null-co://
 $QEMU_IMG info 'json:{"driver": "nbd", "path": 42}' | grep '^image'
 rm -f 42