fix: use sparse loopback file instead of qcow2

- no nbd module in kata so can't use qemu-nbd

.travis.yml

@@ -7,5 +7,4 @@ addons:
       - jq
       - kmod
 script:
-  - sudo apt -y install linux-modules-$(uname -r)
   - ./test.sh

tests/drive.sh

@@ -4,12 +4,9 @@ function test_partitioning()
 {
     local img
     img=$(mktemp -u)
-    qemu-img create -f qcow2 "$img" 16G
-    sudo modprobe nbd
-    sudo qemu-nbd -c /dev/nbd0 "$img"
-    sudo setup_drive /dev/nbd0
-    sudo parted -ms /deb/nbd0 -- print
-    sudo qemu-nbd -d /dev/nbd0
+    dd if=/dev/zero of="$img" bs=1M seek=16384 count=0
+    sudo setup_drive "$img"
+    sudo parted -ms "$img" -- print
     rm -f "$img"
 }