Hi,
this is patch to add maxvcpus support to xen xm command. It's using 
vcpu_avail bitmask and sets the number of vcpus to maxvcpus if present. 
If it's not present, old behavior is preserved.

In domain config file you can define it as follows:

  maxvcpus = 4
  vcpus = 2

this automatically sets vcpus to 4 and corresponding bitmask to present 
2 vcpus in the guest with option to increase it up to 4 vcpus. If 
maxvcpus is not present, the old behavior for vcpus is preserved.

Thanks,
Michal