qemu: prepare secret for the graphics upfront

qemu: prepare secret for the graphics upfront

Instead of hardcoding the TLS creds alias in
qemuBuildGraphicsVNCCommandLine, store it
in the domain private data.

Given that we only support one VNC graphics
and thus have only one alias per-domain,
this is overengineered, but it will allow us
to prepare the secret upfront when we start
supporting encrypted server TLS keys.

Note that the alias is not formatted anywhere
since we won't need to access it after domain
startup.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>