qemu.py: Don't set _popen=None on error/shutdown

Keep the Popen object around to we can query its exit code later.

To keep the existing 'self._popen is None' checks working, add a
is_running() method, that will check if the process is still running.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170526181200.17227-2-ehabkost@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
