qmp: Check for returned data from __json_read in get_events

When QEMU process aborts and socket is closed, qmp client will not
detect it. When this happens, some qemu-iotests scripts will enter an
endless loop waiting for qmp events.

It's better we raise an exception in qmp.py to catch this and make the
test script stop.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
