virtinst: Fix not to return duplicate entries from all_machine_names()

Since machine's canonical alias can be the same as other machine's
name, current all_machine_names() may return duplicates entries.
This causes the problem that the entry of "virt-5.0" appears twice in
machine selection menu when aarch64 is used.

So, fix this by just not adding the same entry twice.

Signed-off-by: Misono Tomohiro <misono.tm@gmail.com>

virtinst/capabilities.py

@@ -138,13 +138,13 @@ class _CapsGuest(XMLBuilder):
     def all_machine_names(self, domain):
         """
         Return all machine string names, including canonical aliases for
-        the guest+domain combo
+        the guest+domain combo but avoiding duplicates
         """
         mobjs = (domain and domain.machines) or self.machines
         ret = []
         for m in mobjs:
             ret.append(m.name)
-            if m.canonical:
+            if m.canonical and m.canonical not in ret:
                 ret.append(m.canonical)
         return ret