feat(instances): the Migrate tab marks hosts it cannot use and files in the way

The Migrate tab offered every other host alike, and its plan showed
neither a file of the same name in a destination pool nor the disks of
the VM's next start, which the job then refused. Now:

- When the tab opens, the hosts are checked first: one that cannot be
  reached, or the VM's own host under another record (the same libvirt
  host UUID), is shown with why and cannot be chosen, and the disks of
  the first one left are checked. The job refuses the VM's own host
  under another name too.
- A pool of the destination that has a file of a disk's name is shown
  but cannot be chosen for it; a disk no pool can take refuses the form.
- The next start's file disks the migration does not carry are listed;
  one not on storage both hosts share refuses the form, as the job does.

vrtManager/instance.py

@@ -276,6 +276,29 @@ class wvmInstances(wvmConnect):
                          "refusal": self.copy_refusal(disk, volume)})
         return rows
 
+    def migration_plan(self, conn, live_xml, next_xml):
+        """The disks of the Migrate tab for a VM coming here (disk_plan of
+        live_xml), each with the pools here it can be copied into, as
+        disk_copies takes them: {free_pools, default} added, and the file
+        disks of next_xml (the next start's definition) that the migration
+        does not carry, which must be on shared storage."""
+        here = self.file_pools()
+        for pool, directory in here.values():
+            pool.refresh(0)
+        disks = self.disk_plan(conn, live_xml)
+        for disk in disks:
+            file = os.path.basename(disk["path"])
+            # a copy never replaces a file: a pool that has one of its name cannot take it
+            disk["free_pools"] = [name for name, (pool, directory) in sorted(here.items())
+                                  if self._find_volume(pool, file) is None]
+            # a copy goes by default to the pool on the same directory, else the first
+            names = [name for name in disk["free_pools"] if here[name][1] == os.path.dirname(disk["path"])]
+            names = names or disk["free_pools"]
+            disk["default"] = names[0] if names else None
+        carried = {disk["path"] for disk in disks}
+        pending = [row for row in self.disk_plan(conn, next_xml) if row["path"] not in carried]
+        return disks, pending
+
     @staticmethod
     def copy_refusal(disk, volume):
         """Why the disk (its volume on the source, None if no pool has it) cannot be copied, or None."""