KVM: MTRR: introduce fixed_mtrr_segment table

This table summarizes the information of fixed MTRRs and introduce some APIs
to abstract its operation which helps us to clean up the code and will be
used in later patches

Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
[Change range_size to range_shift, in order to avoid udivdi3 errors.
 - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>