qemu: Introduce qemuDomainSecretIV

qemu: Introduce qemuDomainSecretIV

Add the data structure and infrastructure to support an initialization
vector (IV) secrets. The IV secret generation will need to have access
to the domain private master key, so let's make sure the prepare disk
and hostdev functions can accept that now.

Anywhere that needs to make a decision over which secret type to use
in order to fill in or use the IV secret has a switch added.

Signed-off-by: John Ferlan <jferlan@redhat.com>