x86/vmx: Internal cleanup for MSR load/save infrastructure

x86/vmx: Internal cleanup for MSR load/save infrastructure

 * Use an arch_vmx_struct local variable to reduce later code volume.
 * Use start/total instead of msr_area/msr_count.  This is in preparation for
   more finegrained handling with later changes.
 * Use ent/end pointers (again for preparation), and to make the vmx_add_msr()
   logic easier to follow.
 * Make the memory allocation block of vmx_add_msr() unlikely, and calculate
   virt_to_maddr() just once.

No practical change to functionality.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>