KVM: MTRR: introduce fixed_mtrr_addr_* functions

Two functions are introduced:
- fixed_mtrr_addr_to_seg() translates the address to the fixed
  MTRR segment

- fixed_mtrr_addr_seg_to_range_index() translates the address to
  the index of kvm_mtrr.fixed_ranges[]

They will be used in the later patch

Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
[Adjust for range_size->range_shift change. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>