Add ability to register callback for virCommand dry run

Add ability to register callback for virCommand dry run

To allow for fault injection of the virCommand dry run,
add the ability to register a callback. The callback will
be passed the argv, env and stdin buffer and is expected
to return the exit status and optionally fill stdout and
stderr buffers.

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