docs: Clarify host-model, host-passthrough differences
docs: Clarify host-model, host-passthrough differences

Based on review feedback from kashyap.

Change-Id: Ief5debad29c10a0903f70d68d5bee21b4c1424b0
Signed-off-by: Stephen Finucane <4725dc7f3632bf126d9fbdc30c347e91e8d2fe16@redhat.com>

openstack/nova,openstack/nova,mahak/nova,mahak/nova,mahak/nova,openstack/nova