qemu.py: cleanup redundant calls in launch()

Now that shutdown() is guaranteed to always execute self._load_io_log()
and self._post_shutdown(), their calls in 'except' became redundant and
we can safely replace it by a call to shutdown().

Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Amador Pahim <apahim@redhat.com>
Message-Id: <20180122205033.24893-6-apahim@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>