x86/MTRR: consolidation

x86/MTRR: consolidation

- use a single set of manifest constants (the ones from msr-index.h)
- drop unnecessary MSR index constants
- get hvm_msr_{read,write}_intercept() in line with the rest of the
  MTRR emulation code regarding the number of emulated MTRRs
- remove use of hardcoded numbers where expressions can be used (at
  once serving as documentation)
- centrally check mtrr_state.have_fixed in get_fixed_ranges(), making
  unnecessary the cpu_has_mtrr check in mtrr_save_fixed_ranges

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>