Fix initialization of current vcpus in libxl driver

Fix initialization of current vcpus in libxl driver

The cur_vcpus member of struct libxl_domain_build_info was incorrectly
initialized to the number of vcpus, when it should have been interpreted
as a bitmap, where bit X corresponds to online/offline status of vcpuX.

To complicate matters, cur_vcpus is an int, so only 32 vcpus can be
set online.  Add a check to ensure vcpus does not exceed this limit.

V2: Eric Blake noted a compilation pitfal when '1 << 32' on an int.
    Account for vcpus == 32.