util: require command args to be non-NULL

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>