Description of problem:
Attach console to a guest which already has console attached would cause race contention, so that both consoles of the guest would get stuck. It would make sense to add a file lock to prevent access to console of a guest when the guest already has a console attached, either in xend or in xenconsole. 

Version-Release number of selected component (if applicable):
xen-3.0.3-131.el5

How reproducible:
Always.

Steps to Reproduce:
1. Start a pv guest and attach console to the guest via xm console.
2. In another terminal, attach the console to the same guest via xm console.
3.
  
Actual results:
The both consoles get stuck, and print strange outputs.

Expected results:
xend or xenconsole should prevent attaching console to a guest which already has a console attached. A warning should be printed so that customers could know that it is not a supported behaviour.

Additional info:
Upstream also have such issues, although they support multiple consoles for pv guest. When you attach the same console(with the same console id) more than once, the consoles would get stuck.