qemu: domain: Inline qemuDomainStorageSourceHasAuth

qemu: domain: Inline qemuDomainStorageSourceHasAuth

The iSCSI hostdev code doesn't require the check for the empty drive
and the check for the protocol because those are already guaranteed at
that point.

In qemuDomainSecretStorageSourcePrepare we don't need to check the
network disk type either as it's now guaranteed by the definition
validator.

Thus both callers can simply check whether src->auth is present and the
helper can be removed.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>