iotests: rewrite 192 to use _launch_qemu to fix LUKS support

The LUKS driver requires extra args to QEMU to setup passwords.
The _launch_qemu function takes care of this, so convert the
test to use this function and use correct -drive syntax

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20170901105434.3288-2-berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
