Don't allow to add more than 4 partitions on msdos disklabels

Signed-off-by: Vojtech Trefny <vtrefny@redhat.com>

blivetgui/blivetgui.py

@@ -303,6 +303,13 @@ def _allow_add_device(self, parent_device, parent_device_type):
                 "missing PVs.").format(name=parent_device.name)
             return (False, msg)
 
+        if parent_device.isDisk and parent_device.format and parent_device.format.type == "disklabel":
+            disk = parent_device.format.partedDisk
+            if disk.primaryPartitionCount >= disk.maxPrimaryPartitionCount:
+                msg = _("Disk {name} already reached maximum allowed count of primary partitions " \
+                        "for {label} disklabel.").format(name=parent_device.name, label=parent_device.format.labelType)
+                return (False, msg)
+
         return (True, None)
 
     def add_partition(self, _widget=None, btrfs_pt=False):