VirtualDisk: Try to append targets, not just use first free (bz #905439)

This used to happen:

- create VM with cdrom, cdrom gets hdc
- customize before install
- add new cdrom, gets target hda or hdb (first free target)
- new cdrom now has priority in the boot order

Change the logic to try to append targets first, and if there isn't
any space left, use the first free one. This may cause other issues
but we'll just have to wait and see.

tests/xmlconfig.py

@@ -1046,6 +1046,14 @@ class TestXMLConfig(unittest.TestCase):
         self.assertEquals("zz", VirtualDisk.num_to_target(27 * 26))
         self.assertEquals("aaa", VirtualDisk.num_to_target(27 * 26 + 1))
 
+        disk = virtinst.VirtualDisk(utils.get_conn())
+        disk.bus = "ide"
+
+        self.assertEquals("hda", disk.generate_target([]))
+        self.assertEquals("hdb", disk.generate_target(["hda"]))
+        self.assertEquals("hdc", disk.generate_target(["hdb", "sda"]))
+        self.assertEquals("hdb", disk.generate_target(["hda", "hdd"]))
+
     def testFedoraTreeinfo(self):
         i = utils.make_distro_installer(
                                 location="tests/cli-test-xml/fakefedoratree")

virtinst/devicedisk.py

@@ -827,13 +827,28 @@ class VirtualDisk(VirtualDevice):
         @rtype C{str}
         """
         prefix, maxnode = self.get_target_prefix()
+        skip_targets = [t for t in skip_targets if t and t.startswith(prefix)]
+        skip_targets.sort()
 
-        for i in range(1, maxnode + 1):
-            gen_t = prefix + self.num_to_target(i)
-            if gen_t not in skip_targets:
-                self.target = gen_t
-                return self.target
+        def get_target():
+            first_found = None
 
+            for i in range(1, maxnode + 1):
+                gen_t = prefix + self.num_to_target(i)
+                if gen_t in skip_targets:
+                    skip_targets.remove(gen_t)
+                    continue
+                if not skip_targets:
+                    return gen_t
+                elif not first_found:
+                    first_found = gen_t
+            if first_found:
+                return first_found
+
+        ret = get_target()
+        if ret:
+            self.target = ret
+            return ret
         raise ValueError(_("No more space for disks of type '%s'" % prefix))
 
 VirtualDisk.register_type()