tests: add vol-qcow2-zerocapacity test to storagevolxml2argvtest

Add a testcase for the previous change to ensure zero capacity volumes can be
defined without a backing store.
Signed-off-by: NChris J Arges <chris.j.arges@canonical.com>

tests/storagevolxml2argvdata/qcow2-zerocapacity.argv

+qemu-img create -f qcow2 -o compat=0.10  0K

tests/storagevolxml2argvtest.c

@@ -284,6 +284,9 @@ mymain(void)
     DO_TEST("pool-dir", "vol-qcow2-nocapacity",
             "pool-dir", "vol-file",
             "qcow2-nocapacity-convert-prealloc", flags, FMT_OPTIONS);
+    DO_TEST("pool-dir", "vol-qcow2-zerocapacity",
+            NULL, NULL,
+            "qcow2-zerocapacity", 0, FMT_COMPAT);
     DO_TEST_FULL(false, VIR_VOL_XML_PARSE_OPT_CAPACITY,
                  "pool-dir", "vol-qcow2-nocapacity-backing", NULL, NULL,
                  "qcow2-nocapacity", 0, FMT_OPTIONS);

tests/storagevolxml2xmlin/vol-qcow2-zerocapacity.xml

+<volume>
+  <name>OtherDemo.img</name>
+  <target>
+    <format type="qcow2"/>
+  </target>
+  <capacity>0</capacity>
+</volume>