Make dom0_enforce_cpus() use vcpu_hotplug rather than directly modifying the sysfs...

Make dom0_enforce_cpus() use vcpu_hotplug rather than directly modifying the sysfs entries.
Directly modifying the sysfs entries causes the xenstore state of
a cpu's availability to be incorrect.  Also slightly modify the
dom0-cpus description in the xend-config.  Rather than specifying which
dom0 vcpus are to be used, it is now a target of how many vcpus to use
as pinvcpu ops are the preferred method for setting which physical cpu a
vcpu uses.
Signed-off-by: Ryan Harper <ryanh@us.ibm.com>
Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>