Add --source-format argument to virsh pool-define-as and pool-create-as

Add --source-format argument to virsh pool-define-as and pool-create-as

When creating pools from dedicated disks, the existing pool-define-as
and pool-create-as commands are a bit non-optimal.

Ideally, a person would be able to specify all of the required options
directly on the command line instead of having to edit the XML.

At the moment, there is no way to specify the format type (ie gpt) so it
gets included in the XML the pool is constructed with.

Please find attached a simple (tested) patch to add an optional
"--source-format 'type'" to virsh.  This is patched against current git
master and will apply cleanly.

Also created a Red Hat BZ ticket for this (#597790) for tracking.