Update the "Enable ACRN Over QEMU/KVM" tutorial:
* Remove the steps explaining how to add the Virtio blk driver
to the Service VM kernel. It is now part of the default
configuration
* Add a note to make it more obvious that the tutorial assumes
that the compilation of ACRN and its kernel is done *inside*
the QEMU VM that will serve as the Service VM for ACRN
Signed-off-by: Geoffroy Van Cutsem <geoffroy.vancutsem@intel.com>