Introduce a new private structure to hold qemu domain auth/secret data.
This will be stored in the qemuDomainDiskPrivate as a means to store the
auth and fetched secret data rather than generating during building of
the command line.

The initial changes will handle the current username and secret values
for rbd and iscsi disks (in their various forms). The rbd secret is
stored as a base64 encoded value, while the iscsi secret is stored as
a plain text value. Future changes will store encoded/encrypted secret
data as well as an initialization vector needed to be given to qemu
in order to decrypt the encoded password along with the domain masterKey.
The inital assumption will be that VIR_DOMAIN_SECRET_INFO_PLAIN is
being used.

Although it's expected that the cleanup of the secret data will be
done immediately after command line generation, reintroduce the object
dispose function qemuDomainDiskPrivateDispose to handle removing
memory associated with the structure for "normal" cleanup paths.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>