HVM vcpu add/remove: parse 'vcpu_avail' to firmware and set up madt

HVM vcpu add/remove: parse 'vcpu_avail' to firmware and set up madt
accordingly

-- currently firmware has got 'vcpus' from xend, this patch add parse
   'vcpu_avail' to firmware;
-- setup madt 'lapic' subitems of processors accoring to vcpus and
   vcpu_avail which finally come from config;

Signed-off-by: Liu, Jinsong <jinsong.liu@intel.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>