iotests: Launch qemu-nbd with -e 42

There is no reason for the qemu-nbd server used for tests not to accept
an arbitrary number of clients. In fact, test 181 will require it to
accept two clients at the same time (and thus it fails before this
patch).

This patch updates common.rc to launch qemu-nbd with -e 42 which should
be enough for all of our current and future tests.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/common.rc

@@ -171,7 +171,9 @@ _make_test_img()
 
     # Start an NBD server on the image file, which is what we'll be talking to
     if [ $IMGPROTO = "nbd" ]; then
-        eval "$QEMU_NBD -v -t -b 127.0.0.1 -p 10810 -f $IMGFMT  $TEST_IMG_FILE >/dev/null &"
+        # Pass a sufficiently high number to -e that should be enough for all
+        # tests
+        eval "$QEMU_NBD -v -t -b 127.0.0.1 -p 10810 -f $IMGFMT -e 42  $TEST_IMG_FILE >/dev/null &"
         sleep 1 # FIXME: qemu-nbd needs to be listening before we continue
     fi