virsh: Add auth options for pool-{create|define}-as

virsh: Add auth options for pool-{create|define}-as

Add 3 new optional options for the pool-create-as and pool-define-as
command in order to define the 3 elements required in order to add
an auth element, such as:

<auth type='chap' username='myuser'>
  <secret usage='libvirtiscsi'/>
</auth>