tests: qemublock: Add test combining authentication and encryption

tests: qemublock: Add test combining authentication and encryption

iscsi and rbd support authentication of the connection. Combine it with
encryption of qcow2.

The top level disk image would generate the following '-drive' cmdline:

-drive file=rbd:rbdpool/rbdimg:id=testuser-rbd:auth_supported=cephx\;none:
            mon_host=host1.example.com\;host2.example.com,
            file.password-secret=node-a-s-secalias,encrypt.format=luks,
            encrypt.key-secret=node-b-f-encalias,format=qcow2,
            if=none,id=drive-dummy
-device virtio-blk-pci,scsi=off,drive=drive-dummy,id=dummy

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>