Update the testing on a model guide.

This patch updates the related README file:
 - replaces the copy-pasted instructions with links to the
   corresponding sub-guides; thus reducing the chance of
   them being not up-to-date.
 - introduces the QEMU setup.

Test: followed the instructions and run tests on QEMU
      successfully.

Change-Id: Icd04a8e4f95a22510841376e41a985b5661e7ce5