Python: close the log file kept by QEMUMachine before reading it

Closing a file that is open for writing, and then reading from it
sounds like a better idea than the opposite, given that the content
will be flushed.

Reference: https://docs.python.org/3/library/io.html#io.IOBase.close


Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20210211220146.2525771-2-crosa@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>