Base: Run SpiceAgent as a user service

This allows it to read/write to the user's clipboard properly. Prior to
this, it would be writing to the Clipboard server running under the
window user, which doesn't impact other users (like anon).

Co-authored-by: Daniel Bertalan <dani@danielbertalan.dev>

Base/etc/SystemServer.ini

@@ -59,9 +59,6 @@ User=root
 WorkingDirectory=/root/
 SystemModes=generate-manpages
 
-[SpiceAgent]
-KeepAlive=false
-
 [LoginServer]
 User=root
 Arguments=--auto-login anon

Base/home/anon/.config/SystemServer.ini

@@ -129,3 +129,6 @@ WorkingDirectory=/home/anon
 
 [CrashDaemon]
 KeepAlive=true
+
+[SpiceAgent]
+KeepAlive=false