- kvm_mtrr_get_guest_memory_type() only checks one page in MTRRs so
   that it's unnecessary to check to see if the range is partially
   covered in MTRR

 - optimize the check of overlap memory type and add some comments
   to explain the precedence
Signed-off-by: NXiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>