Added step to change 'libvirt_type' when running compute on VM

Quite a few people initially try OpenStack by running each node on
a virtual machine. In some cases, either the hypervisor and/or CPU
lack support for hardware acceleration required by KVM on the compute
node. I added a step including how to test for hardware acceleration
support within the virtual machine and change 'libvirt_type' from KVM
to QEMU if the test fails.

Change-Id: I519cdff8c2c1b6a8d1d2b9d5181b205362651b85
backport: havana
Closes-Bug: #1264175