client.shared.test: Add call trace back to log.

When debug qmp_basic_rehl6.py, got following output:
11:54:18 INFO | Running function: qmp_basic_rhel6.run_qmp_basic_rhel6()
11:54:18 ERROR| Test failed: TestFail: 'qemu' key is not of type '<type 'unicode'>', it's '<type 'dict'>'
11:54:18 ERROR| /usr/code/autotest/client/tests/virt/virttest/video_maker.py:195: DeprecationWarning: gst.Bin.add_many() is deprecated, use gst.Bin.add()
11:54:18 ERROR|   pipeline.add_many(source, decoder, encoder, container, output)

11:55:08 ERROR| child process failed
11:55:10 INFO | 		FAIL	virt.qemu.Host_RHEL.6.2.repeat1.qcow2.virtio_blk.smp2.virtio_net.RHEL.6.4.x86_64.qmp_basic_rhel6	virt.qemu.Host_RHEL.6.2.repeat1.qcow2.virtio_blk.smp2.virtio_net.RHEL.6.4.x86_64.qmp_basic_rhel6	timestamp=1370663710	localtime=Jun 08 11:55:10	'qemu' key is not of type '<type 'unicode'>', it's '<type 'dict'>'
11:55:10 INFO | 	END FAIL	virt.qemu.Host_RHEL.6.2.repeat1.qcow2.virtio_blk.smp2.virtio_net.RHEL.6.4.x86_64.qmp_basic_rhel6	virt.qemu.Host_RHEL.6.2.repeat1.qcow2.virtio_blk.smp2.virtio_net.RHEL.6.4.x86_64.qmp_basic_rhel6	timestamp=1370663710	localtime=Jun 08 11:55:10
11:55:10 INFO | END GOOD	----	----	timestamp=1370663710	localtime=Jun 08 11:55:1

It is difficult for us to debug the fail.

Signed-off-by: Feng Yang <fyang@redhat.com>