Use the volume UUID to search for the GRUB config in btrfs partitions

For UEFI installs, a minimal GRUB config file is created in the EFI System
Partition that is used to load the main config file that is located in the
/boot/grub2 directory. The partition that contains the latter, is found by
the minimal config searching a device with a given filesystem UUID.

But for this to work the stage2 device must have a filesystem UUID set and
blivet doesn't set this for partitions formatted with a btrfs filesystem.

Because each btrfs volume will have its own UUID, the UUID of the volume
that's mounted according to /etc/fstab must be used to search the device.

Resolves: rhbz#1930567

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>

pyanaconda/modules/storage/bootloader/efi.py

@@ -172,7 +172,13 @@ def write_config(self):
 
         with open(config_path, "w") as fd:
             grub_dir = self.config_dir
-            fs_uuid = self.stage2_device.format.uuid
+            if self.stage2_device.format.type != "btrfs":
+                fs_uuid = self.stage2_device.format.uuid
+            else:
+                fs_uuid = self.stage2_device.format.vol_uuid
+
+            if fs_uuid == None:
+                raise BootLoaderError("Could not get stage2 filesystem UUID")
 
             grub_dir = util.execWithCapture("grub2-mkrelpath", [grub_dir],
                                             root=conf.target.system_root)