tests: Add a tester for HMP commands

HMP commands do not get any automatic testing yet, so on certain
QEMU machines, some HMP commands were causing crashes in the past.
Thus we should test HMP commands in our test suite, too, to avoid
that such problems creep in again in the future.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1493097407-20482-1-git-send-email-thuth@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>