container: Fix failure with defining a new VM

- We write to a file and immediately this file is consumed by the virsh command.
If the file is not flushed, the virsh command will see an empty file.

Tests:

- Without the change, virsh fails with an error message that the definition file
is empty. With the change, the failure goes away.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

container

@@ -1392,6 +1392,7 @@ class VM(Machine):
 
         with tempfile.NamedTemporaryFile() as file_handle:
             file_handle.write(domain_xml.encode())
+            file_handle.flush()
             logger.info('Running `virsh define %s`', file_handle.name)
             self._virsh(['define', file_handle.name],
                         stdout=subprocess.DEVNULL)