util: require command args to be non-NULL

The virCommand APIs do not expect to be given a NULL value for an arg
name or value. Such a mistake can lead to execution of the wrong
command, as the NULL may prematurely terminate the list of args.
Detect this and report suitable error messages.

This identified a flaw in the storage test which was passing a NULL
instead of the volume path. This flaw was then validated by an incorrect
set of qemu-img args as expected data.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

tests/storagevolxml2xmlin/vol-qcow2-zerocapacity.xml

@@ -1,6 +1,7 @@
 <volume>
   <name>OtherDemo.img</name>
   <target>
+    <path>/var/lib/libvirt/images/OtherDemo.img</path>
     <format type="qcow2"/>
   </target>
   <capacity>0</capacity>