virttest: Handle AttributeError exception in store_vm_register() function.

We may got following error when monitor not ready:
07/05 13:11:07 ERROR| threading:0545|   File "/usr/lib64/python2.6/threading.py", line 532, in __bootstrap_inner
07/05 13:11:07 ERROR| threading:0545|     self.run()
07/05 13:11:07 ERROR| threading:0545|   File "/usr/lib64/python2.6/threading.py", line 484, in run
07/05 13:11:07 ERROR| threading:0545|     self.__target(*self.__args, **self.__kwargs)
07/05 13:11:07 ERROR| threading:0545|   File "/usr/local/staf/test/RHEV/kvm/autotest-devel/client/tests/virt/virttest/env_process.py", line 1092, in _store_vm_register
07/05 13:11:07 ERROR| threading:0545|     stored_log = store_vm_register(vm, vr_filename)
07/05 13:11:07 ERROR| threading:0545|   File "/usr/local/staf/test/RHEV/kvm/autotest-devel/client/tests/virt/virttest/env_process.py", line 1052, in store_vm_register
07/05 13:11:07 ERROR| threading:0545|     output = vm.monitor.info('registers', debug=False)
07/05 13:11:07 ERROR| threading:0545| AttributeError: 'NoneType' object has no attribute 'info'

Signed-off-by: Feng Yang <fyang@redhat.com>
virttest.env_process: Use vm.instance in counter and vm_register_error_count in _store_vm_register()

Now we use vm.name in  counter and vm_register_error_count in _store_vm_register().
It could not work well in migration mode, which two vms have same name.

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #1762 from FengYang/AttributeError

Update  _store_vm_register()