Implement auto-increment partition numbers

- Remove the (1-4) since GPT partition doesn't have a limit
- Calculate the lastpartition value and handle the case where
  we don't have any at first

Signed-off-by: Ryan Harper <ryan.harper@canonical.com>

subiquity/models/blockdev.py

@@ -142,7 +142,8 @@ class Blockdev():
 
     @property
     def lastpartnumber(self):
-        return self.disk.lastPartitionNumber
+        return self.disk.lastPartitionNumber if \
+            self.disk.lastPartitionNumber > 0 else 0
 
     def delete_partition(self, partnum=None, sector=None, mountpoint=None):
         # find part and then call deletePartition()

subiquity/ui/views/filesystem.py

@@ -40,8 +40,9 @@ class AddPartitionView(WidgetWrap):
         self.signal = signal
         self.selected_disk = self.model.get_disk(selected_disk)
 
-        self.partnum = IntegerEditor(caption="Partition number (1-4): ",
-                                     default=1)
+        self.partnum = IntegerEditor(
+            caption="Partition number: ",
+            default=self.selected_disk.lastpartnumber + 1)
         self.size_str = _humanize_size(self.selected_disk.freespace)
         self.size = StringEditor(
                 caption="Size (max {}): ".format(self.size_str))