anaconda: lower minimum required disk space

Indeed 50GB is advisable minimum to have usable system, but do not rule
out some corner cases/test installs.

anaconda/pyanaconda/installclasses/qubes.py

@@ -60,7 +60,7 @@ class InstallClass(BaseInstallClass):
         for autoreq in storage.autoPartitionRequests:
             if autoreq.mountpoint == "/":
                 autoreq.maxSize=None
-                autoreq.requiredSpace=50*1024
+                autoreq.requiredSpace=24*1024
             if autoreq.mountpoint == "/home":
                 storage.autoPartitionRequests.remove(autoreq)