libxl: fix bug the number of running vcpu inconsistent with config file

libxl: fix bug the number of running vcpu inconsistent with config file

The following patch will fix the bug 1825. http://bugzilla.xen.org/bugzilla/show_bug.cgi?id=1825

When using memcpy to update vcpu_online, the high bits of vcpu_online
may unmodified if the size of avail_vcpus less than vcpu_online. So we
need to clear it before the memory copying.

Signed-off-by: Yang Zhang <yang.z.zhang@Intel.com>
Acked-by: Ian Campbell <ian.campbelL@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>