virt-manager: Future-proof disk 'removable' check box

Currently only QEMU is supported so we only show the check box when
it's used. The future-proofing is that we'll show it for an explicitly
set non-default value, even for hypervisors we don't think support it.

virtManager/details.py

@@ -2810,6 +2810,8 @@ class vmmDetails(vmmGObjectUI):
                                          self.conn.is_test_conn()))
         if removable is None:
             removable = False
+        else:
+            can_set_removable = True
 
         if addr == "spapr-vio":
             bus = "spapr-vscsi"